Factors that could influence the repeatability and individuality of marks in the comparison of firearms: A literature review

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Factors that could influence the repeatability and

individuality of marks in the comparison of

firearms: A literature review

Master Forensic Science

Jorit Delen

12345393

Supervisor

Martin Baiker

Examiner

Erwin Mattijssen

January 2020

Abstract

Repeatability and individuality are important concepts in firearm com-parison. Repeatability means a firearm will produce similar marks on every fired bullet and cartridge case, and individuality means a firearm will leave unique markings that can only be traced back to that specific firearm. Several factors that might influence these concepts have been studied based on 31 articles from the available literature. Although both were found to slightly influence the identification process, repeated firing and subclass characteristics were found to not have a significant effect. The brand of firearm used was found to be an important factor, with some brands (like Hi-Point) performing significantly worse. Parameters of the ammunition like diameter and hardness were also found to have an effect, however almost all papers did not go into depth about what the exact influence of these factors was. Finally, external effects like damage to the bullets and rust on the firearm were found to have some effect, but comparison was often still possible.

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1 Introduction

Forensic firearm examination is a very important discipline within forensic sci-ence. Crimes containing a firearm or often very severe and violent, sometimes with lethal results. If a crime occurs with a firearm, a question often asked to the forensic firearm examiner is ”Was this bullet or cartridge case fired by this firearm?”. To answer this question, the examiner compares marks on test shots fired through the gun in question with marks on the found bullets and cartridge cases.

When the trigger of a firearm is pulled, the following actions happen that result in measurable traces: The firing pin hits the primer, which results in the firing pin impression. The primer detonates a small charge, which causes the powder in the cartridge to burn up and become a mixture of gasses. These gasses expand rapidly which pushes the bullet forward through the barrel. Gun barrels are ri-fled, which cause the bullet to spin, increasing stability of the bullet trajectory. These rifled grooves leave striations on the bullet, known as land marks. The expanding gasses also push the cartridge backwards, into the breech face. This results in the creation of breech face marks on the back end of the cartridge case. In most semiautomatic pistols, the slide also gets pushed backwards, and the rear end of the barrel will move slightly downwards. This combined with the cartridge case being pushed against the firing pin hole can lead to the creation of aperture shear marks on the cartridge case. If the firing pin remains in contact with the primer during this moment, this will also cause the creation of a firing pin drag marks. Finally, the cartridge case will be extracted from the chamber by the extractor and ejected through the ejector. In some types of firearm this can lead to ejector marks and extractor marks on the cartridge. There are other minor marks (like the anvil mark, magazine lip mark), however these do not appear in most firearms, are rarely used in cases and rarely show up in literature. This literature review will not cover these marks.

A picture of some of the marks described can be found in Figure 1

The characteristics that result from firing a firearm can be split into three cat-egories:

• Class characteristics are caused by the design of the firearm, and will be the same for all firearms of that brand or model. An example is the general shape of the firing pin.

• Subclass characteristics are caused by the manufacturing process and can be the same for some firearms. An example is a mark left on cartridge cases fired by all firearms whose barrels were rifled by the same tool. • Individual characteristics, also known as accidental characteristics, are

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Figure 1: A cartridge case and bullet with the following marks: Firing pin impression (A), Ejector mark (B), Breech face mark (C), Firing pin drag mark (D), Land marks (E). Pictures taken from Ultra Electronics Forensic Technology, Inc. (http://ultraforensictechnology.com/pubs#bro)

caused by random imperfections during the manufacturing process and (should be) different for every firearm. An example is the exact breech face impression.

The researcher mainly wants to focus on the individual characteristics, as these can give the most evidence if they match.

2 Research question

Firearm identification is based on two important assumptions, the assumption of individuality and the assumption of repeatability. In this case repeatability means that a single firearm will always leave similar marks on bullets and car-tridge cases. If the marks were to change too much too quickly, then bullets from a specific gun could not be matched with bullets fired later from the same gun, which is a vital part of the identification process. The second assumption, individuality, means that every gun leaves a distinct trace on the bullet and cartridge case that are fired with it. If this was not the case then the evidence delivered by firearm examination would be a lot less strong, as two guns could deliver (almost) the same marks

There are several factors that could influence these assumptions, like the mate-rial of the cartridge, the brand of the firearm or external effects like damage on the bullet. In this literature review my research question will be as follows:

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What are (possible) factors that could influence the repeatability and individuality of traces on bullets and cartridge cases fired by a firearm, and how do they affect these?

3 Search strategy

To answer this question I did a survey of the available literature of this subject. I started the search with going through the online library of my university, especially the following journals:

• AFTE Journal

• Forensic Science International • Journal of Forensic Sciences

The search started with looking through these journals using keywords such as ”repeatability” and ”individuality”, and words based on the different traces such as ”firing pin impression” and ”aperture shear mark”. In doing this I focused more on recent articles. After reading these articles I found some other useful articles that came up often, either through references in multiple papers or articles written by authors that were named often. Finally, my supervisor gave me access to the final project of a former intern (N. Brand, 2017) at his place of employment, the NFI (Netherlands Forensic Institute). From all these articles I made a final selection of the 31 most relevant articles that will be discussed in the next sections. A complete table of the key facts of all articles can be found in Appendix A.

4 Results and discussion

In this section I will summarize the results based on possible factors. I have split this up into two sub-chapters, one dealing with the repeatability of traces and the other with the individuality of traces.

4.1 Repeatability

First I will cover repeatability. As mentioned before, the assumption of repeata-bility is that a firearm will produce the same (or a similar enough) trace every time it is fired. In studies done using an automated comparison system this is closely related with the within distribution and variability of a firearm. This is the distribution of the comparison scores of shots taken with the same firearm, and its variability. If repeatability of a firearm is high, the comparison scores of bullets fired by this firearm will be high and the variability low.

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4.1.1 Repeated firing and wear

I will start with studies dedicated to the effect of repeated firing. This could influence repeatability for example through the wear of the barrel changing the grooves, which would lead to a different set of markings being left on the bullet. This can be useful for cases that are not solved immediately where the firearm is only found much later and has been used extensively in the meantime. In 1958, Kirby [1] fired a S&W revolver 900 times. He found the land marks on the bullets to be changing extremely rapidly, even go so far as saying that by the fiftieth bullet: ”If, at this time, an opinion of identification was asked for (...) his opinion would have to be in the nagative [sic]”. All finer striations that were visible in the first shots had been replaced by a completely new set. The author blames the buildup of lead in the barrel as the primary reason for these changes. The breech face markings on cartridge cases showed no major differences over all shots.

For the land marks, a different conclusion was reached by Bachrach in 2006 [3], who fired 200 bullets through 4 barrels of different brands. For all brands there were minor changes noted over the repeated firings, however according to him ”these effects do not appear to be significant enough to prevent cor-rect identification”. This seems to be a more common theme in articles where a single examiner manually compares land marks fired from a single firearm, with other authors and papers like Mikko 2012 and 2013 [12, 13] and Wong 2013 [16] stating similar results. Mikko states that even though there were land marks that changed to such a point that they were not as useful for identifica-tion as they were in the beginning, ”Microscopic examinaidentifica-tion and comparison (...) revealed sufficient matching individual striations to enable the examiners to identify the barrel of origin for each of the bullets”, even after shooting the firearm 20000 times. Wong states a result similar to Bachrach over 1000 test fires, that although there were changes to the land marks, these changes were insignificant for identification purposes.

Cartridge cases appear to be more constant over time. As mentioned before, Kirby 1958 [1] states that breech face marks and firing pin impressions remain similar over all shots. Uchiyama 2008 [4] partly agrees with this. The paper does mention that after seventy shots slight differences in the breech face markings were starting to appear, but these were minor enough that identification was still possible. Sarıbey 2009 [6] used some more statistical methods to analyze this. In their paper they found that none of the analyzed types of markings changed significantly, even over 5000 test fires. The biggest change was found in the length of the extractor marks, but even this was not a significantly large change. Wong 2013 [16] found firing pin aperture shear marks to be inconsis-tent throughout 1000 firings, however this seems to have more to do with the randomness of the trace itself than with the effects of repeated firings. Even then these markings could still be used to correctly identify all but three of the

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cartridge cases.

These conclusions are strengthened by several studies that used more than 30 examiners ([2], [5], [9], [14], [15]). All of these studies used several firearms, all fired a number of times ranging from 120 per barrel in Brundage 1998 [2] to 1680 per barrel for Hamby 2009 [5]. The error rates of all these studies was extremely low, for both bullets and cartridge cases. The results ranged from Brundage 1998, who reports no errors at all, to Fadul Jr. 2013 [15] reporting an error rate that is ”Statistically different from zero, (...) [but] no larger than 0.012, or 1.2%”.

Automated systems were also found to operate properly when dealing with re-peated firings. Grom 2012 [7] found that the IBIS BrassTRAX-3D system could still properly identify almost all cartridge cases to a firearm fired 500 times, even when comparing to 909 other cartridge cases with the same class characteristics. To quote: ”The individual characteristics of the breech face and firing pin did not change significantly, leaving similar features that could be correctly asso-ciated by IBIS during the correlation process”. Kirk 2017 [23] found similar results. In firing 24 firearms 200 times each, they found that when using the IBIS Heritage system for all firearms the within distribution was significantly different than the between distribution. This implies identification should still be possible after repeated firing. A slightly different conclusion was reached in Zhang 2017 and 2018 [24, 27], who used the ABIS Evofinder system to compare 3070 cartridge cases fired from the same gun to 325 cartridge cases fired by firearms of the same model. They state that breech face marks and firing pin impressions change significantly over repeated firings, however ”The variation of marks from the same firearm did not exceed the differences between marks of different firearms”, so correct identification was still possible. Christen 2019 [28] used the same system to compare 500 shots from a firearm to 84 bullets from the same type of firearm. They too found that identification would still be possible after the repeated firing, stating ”We could not determine a trend indicating that markings and thus matching results deteriorated with the num-ber of cartridges shot”.

In conclusion, almost all studies found that repeated firing does not influence the ability of the firearms examiner to come to the correct conclusion, and so repeatability was not influenced by this. This was the case both in manual ex-aminations, in studies with multiple examiners, and if automatic systems were used. One exception was Kirby 1958, however after personal correspondence with my supervisor his reasoning for this is that the quality of firearms and bul-lets has increased over the years, and as a result the buildup of lead as stated in that study will not happen with modern firearms.

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Table 1: The brands of firearms that appeared in the literature, and their repeatability. For cartridge cases the mark is specified

Brand References Bullets Cartridge cases

Ruger [2, 3, 5, 16, 18, 23] Very good Very good

(Aperture shear marks, Breech face marks, Firing pin impressions)

Glock [7, 9, 15, 28] Very good Very good

(Breech face marks, Firing pin impressions)

Hi-Point [4, 11, 18] Poor Poor

Beretta [3, 18] Good Good

(Breech face marks)

Taurus [11, 18] Good Good

SIG Sauer [11, 20] - Very good

(Breech face marks, Firing pin impressions) Other [3, 6, 12, 24, 29, 31] Medium - Very good Medium - Very Good

4.1.2 Firearm brand

In this section I will cover the different brands that were used in the studies. Some brands of firearm might reproduce marks better than others. The brand is of course not something that the examiner can influence during the comparison, but the effect of the brand should still be something that the examiner should be aware of. Table 1 shows the different brands, and their repeatability on a scale from very poor to very good. A more in depth description will be given here:

• Ruger firearms showed a very good level of repeatability in several studies. In Bachrach 2006 [3], the used Ruger pistol performed better (i.e. the markings on the bullets changed less) than the used Beretta, Browning and S&W pistols. Brundage 1998 [2], Hamby 2009 [5] and Wong 2013 [16] showed that bullets fired from Ruger pistols could still be identified after many firings, also in experiments with multiple examiners. For cartridge cases the same conclusion was reached by Wong 2013 [16] and Kirk 2017 [23], wherein almost all cartridge cases could be identified and there were significant differences in the within and between distributions.

• Glock pistols also performed very well, albeit slightly worse than Ruger. Fadul Jr. 2011 and 2013 [9, 15] state that the error rate of examiners over repeated shots for bullets fired from Glock pistols is about 0.5%. Christen 2018 [28] showed that the analyzability of bullets from Glock pistol showed no downward trend over repeated firings. For cartridge cases a similar result was found by Grom 2012 [7], with only two of the firing pin impressions not being correctly identified by the system. All of

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this however might be only a recent trend, with several articles (such as Fadul Jr. 2013 and Christen 2018) mentioning that Glock barrels used to be difficult to analyze due to polygonal rifling.

• The repeatability of Hi-Point pistols is a lot lower than the previous brands, especially on land marks. In Uchiyama 2008 [4] the author states that ”The reproducibility of striations was not high throughout these one hundred bullets”, with only three of the nine land marks reproducing well. In both other studies in which the repeatability of this brand is tested (Chumbley 2011 [11] and Mears 2013 [18]) Hi-Point pistols performed less well than pistols of other brands, both in the land marks on bullets and the breech face marks on cartridge cases. In Chumbley 2011 the Hi-Point was significantly outperformed by both Ruger and Taurus pistols, while in Mears 2013 all markings on both bullets and cartridge cases were shown to replicate poorly in comparison to other brands.

• Beretta performed decent in both studies in which pistols of that brand were featured. In Bachrach 2006 [3] the variability of the used Beretta ranked second least, after the Ruger. In Mears 2013 [18], the markings on bullets and cartridge cases fired by Beretta replicated decently well. • Taurus also showed a decent amount of repeatability in both of the studies

in which it was featured. In Chumbley 2011 [11], the Taurus pistol showed worse repeatability in the firing pin impressions than the SIG Sauer, but better markings than the Hi-Point. In Mears 2013 [18], once again the markings on both bullets and cartridge cases replicated decently well for the used Taurus.

• SIG Sauer pistols on the other hand were found to perform very well in both studies they were featured in (Chumbley 2011 [11] and Riva 2014 [20]). In Chumbley 2011 the firing pin impressions of the SIG Sauer pistol performed better than the Taurus and definitely better than the Hi-Point. In Riva 2014 for all tested models of SIG Sauer, the within distribution and between distribution were significantly different. Unfortunately both these studies concern cartridge cases, so no statement can be made about the repeatability of the land marks.

• Under Other are all brands of firearm that were only used in one study. A short summary of their repeatability follows: Outside of the already mentioned Ruger and Beretta, Bachrach 2006 [3] also featured a S&W and a Browning brand firearm. Both of these performed medium well, with their variability being higher than the Ruger and Beretta, but bullets from both firearms were still recognizable after 200 shots. Sarıbey 2009 featured five Turkish brands, all of which performed very well to the point that after 1000 shots the cartridge cases were still identifiable. Mikko 2013 [12] was the only study that used a M240 machine gun, which performed well. Changes were seen on the bullets, however even after 20000 shots the first bullet could still be matched with the last. Zhang 2017 [24] and Dong

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2019 [29] both used the same dataset of cartridge cases fired by Norinco brand firearms, which replicated well. Changes were found in the shots but all cases could still be matched with each other.

It seems that the brand of the firearm does have a noticeable effect on the repeatability of the markings, with Hi-Point being a notable example. However, there were very few studies that actually compared the repeatability of different brands of firearms to each other, so a lot of the conclusions in this table might be based on an unfair comparison of test circumstances. For example, some studies were done using manual comparisons while others were done using automatic systems, and it is debatable whether these results can be compared.

4.1.3 Cartridge material and dimensions

The dimensions and material of the cartridge can also be an important factor. The material of the bullet could influence how well striations are left. The same can be said for the primer material, which could for example influence the fir-ing pin impression and the breech face impression. Generally speakfir-ing in this section shallower striations and impressions imply less repeatability.

In Bachrach 2006 [3], Remington ammunition provides better landmarks than Winchester ammunition, even when both types of ammunition were 9mm cal-iber and 115 grain weight. The author gives hardness of the bullet jacket and diameter of the bullet (although unfortunately they do not mention what ma-terial the bullets in question are made of, or the exact diameters). Hardness of the jacket can influence how well striations are left on the bullet. According to the author, bullet diameter can influence the striations in two ways: First, if the diameter is small then there will not be much contact between the grooves of the barrel and the bullet, which will leave worse striations. Second, if the diameter is large, more pressure will build up behind the bullet thus pushing it with higher force, leading to deeper striations. Uchiyama 2008 [4] came to some similar conclusions in their study where they analyzed five different types of ammunition, including three types that only differ in grain (Speer gold dot 9mm Luger 115, 124 and 147 grain). As in the previous study, they stated that there was a strong correlation between bullet diameter and quality of striations near the trailing edges. They also state that weight coupled with muzzle veloc-ity (which are connected in the sense that higher weight in grain causes a lower muzzle velocity) is a major factor in the quality of striations.

For cartridge cases, Chumbley 2012 [11] tested a modified version of the fir-ing pin impression for 10 different types of ammo. They found that for different types of ammunition the firing pin impression differed significantly. However, as opposed to the previous studies, they did not find any primary parameter responsible for this: ”No consistent tends were observed as a function of ei-ther primer material, type or hardness, and/or cartridge case material”. One exception is the presence of lacquer on cartridge cases, which influences the

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impression to such a point that identification through the firing pin impression is close to impossible. Addinnal 2019 [30] showed that even within the same brand and type of ammunition, the elemental composition can vary. They state that elemental composition is an important factor in the shaping of the firing pin impression, with an higher percentage of softer materials like zinc leading to a deeper and wider firing pin impression. For some types of ammunition, the correlation scores were very low, and this could not only be attributed to elemental composition. However other factors are not covered in this study. Manzalini 2019 [31] agrees that elemental composition is a major factor in how analyzable the striations of a bullet are, with softer materials giving better markings. However, this could also work in the negative, as one bullet that was very low in antimony (and thus very soft) showed so many markings that identification was impossible. They also come with at warning that even within the same brand of ammunition, many differences can appear in the elemental composition. This to the point that for different lots of the same type of ammu-nition, one was made of a lead-tin alloy, and the other of a lead-antimony alloy. In conclusion, cartridge parameters can influence the repeatability of markings. For bullets, hardness, diameter and weight were found to be most impactful, while for cartridge cases no main parameter could be found with the exception of the presence of lacquer, and possibly the elemental composition, however this has not been studied yet to the point that meaningful conclusions can be drawn from this. Unfortunately, almost all studies of this chapter did not go into detail about how exactly these parameters influence the process of identification. Un-til all these factors are better understood, when possible the examiner should try to find a type of ammunition that is as close as possible as the bullet or cartridge case in question (which will be easier for cartridge cases as these often contain information about brand and type). This can prevent these factors from influencing the analysis.

4.1.4 External effects

In most studies, all shots were taken in laboratory settings. However, in real life this is often not the case, so factors like damage to bullets should also be studied. This section covers factors that do not directly influence the repeata-bility of a firearm, but do complicate the analysis.

If a firearm is thrown in a river or other body of water after a shooting, rust can occur. Mears 2013 [18] tested the effectivity of several rust removal methods, and what influence this would have on the repeatability of the firearms in ques-tion. Unfortunately, they found there is no perfect soluques-tion. This study used 10 different types of firearms and 10 different types of rust removal methods, and found that the results are very dependant on whether the firearms were repeating well before the rusting. If the same type of firearm was used for all methods, than a better comparison could have been made. Ignoring that and

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other variables that are in play, the author recommends using either soda blast-ing or Rust ReleaseTM

In real cases the bullets and cartridge cases are sometimes only found after a while, and have been exposed to the elements in the meantime. To test this effect, Pollut 2016 [22] buried fired cartridge cases in moist soil for up to 12 weeks, to check whether that would influence the aperture shear mark. The conclusion was found that it will not, and the mark was still comparable after this time. They found that the only parameter that was significantly impacted is the height skewness distribution, which could be a statistical error.

4.2 Individuality

The second concept introduced was individuality. This means that every firearm should leave a unique set of markings on the fired bullets and cartridge cases. In automated comparison system, this would imply that bullets from different firearms should have a low comparison score.

4.2.1 Subclass characteristics

As mentioned in the introduction, subclass characteristics are characteristics that result from tools used during the manufacturing process. These will then be the same for all firearms produced with that tool. Researchers should be very careful with these, because if these are mistaken for individual character-istics a false identification could be made. A common way to test the effect of these characteristics is to compare bullets and cartridge cases coming from firearms that have been manufactured consecutively with the same tools and so will have the same subclass characteristics. If the markings from these can be easily distinguished, the subclass characteristics will be shown to have little to no effect.

Brundage 1998 [2] fired bullets through 10 consecutively manufactured bar-rels and send them to 30 different examiners. No errors were reported although 2 bullets were deemed inconclusive. In a study with a similar setup to the previous Hamby 2009 [5] also found no errors and only very few (in this case 8) inconclusive. Fadul Jr. 2011 and 2015 [9, 15] found an average error rate of only 0.5% in two studies with similar setups with 183 and 156 participants, respectively.

Bachrach 2008 [3] on the other hand found that individuality could only be achieved for some brands of firearm. Ruger, Beretta and S&W were very clear, Taurus and Browning were not as clear and SIG Sauer, Hi-Point and Bryco were not clear at all. This could be due to the algorithm used though, as this algorithm was not used again in other studies. It is also not entirely clear how big of a factor subclass characteristics were in this situation, or if the system used just had problems in general.

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Table 2: The brands of firearms that appeared in the literature, and their individuality. For cartridge cases the mark is specified

Brand References Bullet individuality Cartridge case individuality

Glock [9, 15, 19, 21, 26, 28] Very good Very good

(Aperture shear marks)

Ruger [2, 3, 5, 10, 14, 23] Very good Very good

(Breech face marks)

Hi-Point [3, 8] Poor Good

(Breech face marks)

SIG Sauer [3, 20] Poor Very Good

(Breech face marks, firing pin impressions)

Other [3, 17, 29] Medium - Very good Medium - Very good

For cartridge cases, LaPorte 2011 [8] fired cartridge cases through 10 consecu-tively made slides. The breech face marks of all cases could be correctly matched to the firearm they came from and ”No subclass influence is present, (...) any correspondence between toolmarks produced by different slides is purely coinci-dental”. This is confirmed by Weller 2012 [10], who used an automated system to study breech face marks. According to them ”Subclass toolmarks had no mathematically detectable influence in the cross-correlation scores”. Fadul Jr. 2013 [14] found an error rate for cartridge cases that was not significantly dif-ferent from zero.

In conclusion, even in studies where subclass characteristics were present, the used identification processes were still capable of properly identifying (almost) all fired bullets and cartridge cases. This means that although they do influence it slightly, subclass characteristics do not significantly reduce the individuality of firearms. This result appeared in studies with both automatic and manual methods of comparison.

4.2.2 Firearm brand

As in the previous chapter I will cover the different brands of firearms used in the studies in this section. It is possible that some brands leave traces of lower quality which could make it more difficult for the examiner to identify which firearm the bullet or cartridge case came from. Table 2 shows the different brands, and their individuality on a scale from very poor to very good. A more in depth description will be given here:

• Glock firearms were found to perform very well in individuality based studies. Fadul Jr. did two studies, in 2011 and 2015 [9, 15]. In those, an average error rate for the identification of bullets of only 0.5% was found. Glock firearms are known to leave a very clear aperture shear

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mark. Castro 2014 [19] compared aperture shear marks of 1079 different Glock pistols. According to them: ”It was possible to identify each of the two fired cartridge cases to themselves and then compare them against the remaining cases to demonstrate that each set of fired cases had their own individual characteristics”. A similar result was given by Hamby 2016 [21], who manually and automatically compared aperture shear marks of 1632 different Glocks. All cases could manually be identified only to themselves, and automatically an upper limit for an error rate was given at 0.0001%. Brand 2017 [26] also showed that for Glock aperture shear marks the known match distribution and the known non match distribution were significantly different for two types of ammo. The distributions can then provide very strong likelihood ratios. Finally, Christen 2019 [28] showed that for bullets fired from 20 Glock pistols, the within distribution and the between distribution of land mark comparison was significantly different, which implies a high level of individualization.

• Ruger firearms also performed very well. In 1998, Brundage [2] did a study with 10 Ruger barrels, and showed an error rate of 0 with only 2 inconclusive bullets. In the second part of Bachrach 2006 [3], Ruger pistols were among the best performing together with Beretta and S%W. Hamby 2009 [5], Weller 2012 [10] and Fadul Jr. [14] showed that even for ten consecutively manufactured Ruger pistols or slides, all bullets and cartridge cases could still be identified, whether you use manual methods (Weller, Fadul Jr.) or automatic methods (Weller). The effectiveness of automatic methods was strengthened by Kirk 2017 [23], who used an IBIS heritage system to calculate the distributions of 4800 cartridge cases fired by 24 Ruger pistols, and found for all of them a clear distinction in the within and between distributions.

• In the two studies they were featured in, Hi-Point pistols gave mixed re-sults. For bullets and land marks, the system used by Bachrach 2006 [3] had great difficulty with them. The writer claims this is due to Hi-Point barrels having a low overall quality as they are very cheap guns. Another result of this is that Hi-Point pistols have a slightly higher di-mensional tolerance, meaning the barrel has a slightly larger and slightly more inconsistent barrel diameter, which causes the land marks to also be inconsistent and difficult to analyze. On the other hand, in LaPorte 2011 [8], cartridge cases (more specifically breech face marks) showed almost no difficulty in being identified, even when using consecutively manufactured slides.

• SIG Sauer showed some similar results to Hi-Point, but for different rea-sons. In Bachrach 2006 [3], bullets fired from SIG Sauer once agian showed great difficulty in being identified, but this time the reason is that the bar-rel is of a very high quality instead of the lower quality of Hi-Point. The author states that during the production of higher quality barrels the in-sides are often polished as a finishing step. This causes the land marks

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to be very smooth, which can cause them to be difficult to differentiate between different firearms. For cartridge cases this does not appear to be a problem, as Riva 2014 [20] demonstrated. In their study, the author showed that when using an automated system, the cartridge cases gave great differences in the distributions and the associated likelihood ratios had the potential to be very high.

• Once again under Other are brands that were only used in one study. Bachrach 2006 [3] also featured pistols from Beretta, who performed very well, from Taurus and Browning who performed slightly worse but still decently, and from Bryco, who performed on the same level as Hi-Point and SIG Sauer. Sarıbey 2013 [17] analyzed two brands of Turkish guns. They found that between pistols of the same brand/model, class charac-teristics did not differ significantly but individual characcharac-teristics did. This implies good individuality. Finally, Dong 2019 [29] used Norinco brand pistols, and showed that there were significant differences between the within and between distributions for both breech face marks and firing pin impressions.

So the brand of firearm can play a significant role on the individuality of the pistols. Once again, there were only very few studies that directly compared different types of firearm (even less than in section 4.1.2). Most of the analyzed studies were performed under different circumstances, so comparing these as I did might be a bit tricky.

4.2.3 Cartridge material and dimensions

In this section I will cover the effects of ammunition on the individuality of firearms. Some of the results from section 4.1.3 involved both repeatability and individuality and can be carried over to this section. Those results I will only mentioned shortly. In general better striation or impression patterns imply bet-ter individuality.

In Bachrach 2006 [3], Remington brand ammunition provided better landmarks than Winchester brand ammunition. The main important factors given by the author are the hardness of the bullet jacket and the diameter of the bullet. LaPorte 2011 [8] used four types of ammunition, two of which had brass primers and two of which had nickel primers. Unfortunately, all they mention about it is ”The examination revealed that all cartridge cases were marked equally well”, without any explanation or elaboration. Also for cartridge cases, Chumbley 2012 [11] found that the type of ammunition used gave significant differences in the topology of the firing pin impression, however no main responsible pa-rameter could be found. Amongst the papa-rameters tested were primer material, primer type, hardness and cartridge case material. The only exception was the presence of laquer, which made analyzing the firing pin impression close to im-possible. Brand 2017 [26] showed that aperture shear marks on cartridge cases

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of the brand Fiocchi, with a nickel primer, gave less false positives and false neg-atives than Sellier & Bellot, which had a brass primer. Also Fiocchi cartridge cases often showed longer striation patterns than Sellier & Bellot, however no correlation could be found between the length of striations and the correlation score. The writer does not go into detail about other possible factors.

In conclusion, many of the factors that influence the repeatability of the trace also influence the individuality. Examples are bullet hardness or diameter. It is very unfortunate that almost all articles do not go in depth on how these factors influence the comparisons.

4.2.4 External effects

In this section, I will cover some articles that dealt with external effects that are not present in most laboratory settings, but can be present in real life cases. These factors then do not influence the individuality directly, but do complicate the analysis.

Almost all studies analyzed so far have dealt with bullets that are in perfect conditions, collected by bullet catchers. However bullets found on real crime scenes will not always be in perfect shape. They might have been fired into a wall or another hard object, which will have caused deformations. This can especially influence automated systems, as these are often made for cylindrical objects and the deformations can cause the bullets to be unable to be properly scanned. To test this, Bachrach 2006 [3] purposefully shot bullets at hard sur-faces to deform them, to test whether or not the algorithm was still useful. He found that the system could still perform, and there were still differences in the within and between distributions. However these distributions were much closer together which could severely complicate the analysis.

Sometimes after a robbery or shooting, the perpetrators flee in a getaway ve-hicle. To prevent identification through either the vehicle or the firearm, a solution is to burn the getaway vehicle, sometimes with the firearm still inside. To simulate this Collender 2017 [25] set fire to some cars with firearms still inside, and then tried to see if it was still possible to identify bullets form the now burned firearm with bullets fired before it. It was found to be possible to identify cartridge cases fired after the burning to those from before the burning, although the quality of the marks on the cartridge case was reduced. On the other hand, fired bullets could not be identified to their original firearm after the burning. The writer claims damage due to oxidation to the grooves of the barrel as the main reason. Finally, firearms that were left in the footwell performed better than those left on the rear seat, as the footwell reached significantly lower temperatures.

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5 Conclusion and recommendations

Several factors that could possibly influence the repeatability and individuality of marks left by firearms were analyzed. Some, like repeated firing and subclass characteristics, were shown to have very little effect. Others, like the brand of firearm used and the type of ammunition were found to have possible effects on both the repeatability and the individuality. For the brand of firearm, this is of course out of the control of the examiner, but they should still be aware of the possible influences. For the type of ammunition, these effects can be minimized by using a type of ammunition that is as close as possible to the type of bullet or cartridge case in question.

External factors were also considered. Environmental exposure (in this case tested by burial of the bullets in a moist environment for 12 weeks) was found to have little to no effect on the repeatability. Rust (tested by submerging the firearm in salt water for a long time), exposure to high temperatures (tested by leaving a firearm in a burning car) and damage to the bullets (tested by shooting into a hard surface) were found to complicate the analysis, but identification was often still possible. An exception is bullets fired from the guns exposed to high temperatures.

There were in my opinion two major problems with the currently available literature. The first is that almost all papers (with the exception of Riva 2014 and Brand 2017) seem to be based on a yes/no system, where a comparison is either called as a ”match”, ”no match” or ”inconclusive”. In the last few years, more and more countries have switched to systems based on likelihood ratios (LRs), including the Netherlands, where I come from. These systems can usually provide more information than a simple yes/no system, and can help prevent false identifications and false exclusions. Even many papers that used automated systems, which lend themselves very well to the calculation of LRs (for example through dividing the within distribution for a certain value by the between distribution at that value), still gave absolute answers instead (with the logic: ”If the correlation score is above a certain point it will be called a match, while below that score it will be called a non-match”). A recommendation for the future is to perform more studies using LR based systems.

The second problem is that many articles mention certain factors, but then do not go into depth about how these factors could influence the results. This is slightly related to the previous problem, as it was often not necessary to study the exact influences of certain factors. For example, in articles that studied repeatability of a firearm over many shots it was often only mentioned that there were minor changes, but that they did not influence the (yes/no based) comparison. If an LR based system were to be used, these ”minor changes” could still influence the LR and so their effect should be studies. On the other hand, in the last few years more and more articles are being written that do not simply focus on the comparison of fired bullets and cartridge cases, but

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also on how the different factors could affect these (for example Addinnal 2019 and Manzalini 2019). More of these sort of studies must be performed to try to find the exact effects of all unknown factors, to make firearm comparison as objective as possible.

References

[1] Kirby S. (1958). Comparison of 900 consecutively fired bullets and car-tridge cases from a 455 caliber S&W revolver. RCMP Crime Lab 113-126 [2] Brundage D. (1998). The identification of consecutively rifled gun barrels.

AFTE Journal, 30 (3) 438-444

[3] Bachrach B. (2006). A statistical validation of the individ-uality of guns using 3D images of bullets. Retrieved from

https://nij.ojp.gov/library/publications/statistical-validation-individuality-guns-using-3d-images-bullets

[4] Uchiyama T. (2008). Toolmark reproducibility on fired bullets and ex-pended cartridge cases. AFTE Journal, 40 (1) 3-46

[5] Hamby J, Brundage D, Thorpe J. (2009). The identification of bullets fired from ten consecutively rifled 9mm Ruger pistol barrels: A research project involving 507 participants from 20 countries. AFTE Journal, 41 (2) 99-110

[6] Sarıbey A, Hannam A. (2009). An investigation into whether or not the class and individual characteristics of five Turkish manufactured pistols change during extensive firing. Journal of forensic sciences, 54 (5) 1068-1072

[7] Grom T, Demuth W. (2012). IBIS correlation results of cartridge cases collected over the course of 500 firings from a Glock pistol. AFTE Journal, 44 (4) 361-363

[8] LaPorte D. (2011). An empirical and validation study of breechface marks on .380 ACP caliber cartridge cases fired from ten consecutively finished Hi-Point model C9 pistols. AFTE journal, 43 (4) 303-309

[9] Fadul Jr. T. (2011). An empirical study to evaluate the repeatability and uniqueness of striations/impressions imparted on consecutively manufac-tured Glock EBIS gun barrels. AFTE journal, 43 (1) 37-44

[10] Weller T, Zheng A, Thompson R, Tulleners F. (2012). Confocal mi-croscopy analysis of breech face marks on fired cartridge cases from 10 consecutively manufactured pistol slides. Journal of Forensic Sciences, 57 (4) 912-917

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[11] Chumbley L, Kreiser J, Lizotte T, Ohar O, Grieve T, King B, Eisenmann D. (2012). Clarity of microstamped identifiers as a function of primer hardness and type of firearm action. AFTE journal, 44 (2) 145-155 [12] Mikko D, Miller J, Flater J. (2012). Reproducability of toolmarks on 20000

bullets fired through an M240 machine gun barrel. AFTE Journal, 44 (3) 248-253

[13] Mikko D, Miller J. (2013). An empirical study/validation test pertaining to the reproducability of toolmarks on 20000 bullets fired through an M240 machine gun barrel. AFTE Journal, 45 (3) 290-291

[14] Fadul Jr. T, Hernandez G, Stoiloff S, Gulati S. (2013). An empirical study to improve the scientific foundation of forensic firearm and tool mark iden-tification utilizing 10 consecutively manufactured slides. AFTE journal, 45 (4) 376-394

[15] Fadul Jr. T, Hernandez G, Stoiloff S, Gulati S. (2013). Empirical study to improve the scientific foundation of forensic firearm and tool mark identification utilizing consecutively manufactured Glock EBIS barrels. Retrieved from https://nij.ojp.gov/library/publications/empirical-study-improve-scientific-foundation-forensic-firearm-and-tool-mark-0

[16] Wong C. (2013). The inter-comparison of 1000 consecutively-fired 9mm Luger bullets and cartridge cases from a Ruger P89 pistol utilizing both pattern matching and and quantitative consecutive matching striae as criteria for identification. AFTE journal, 45 (3) 267-272

[17] Sarıbey A, Hannam A. (2013). Comparison of the class and individual characteristics of Turkish 7.65 mm Browning/.32 Automatic caliber self-loading pistols with consecutive serial numbers. Journal of forensic sci-ences, 58 (1) 146-150

[18] Mears D. (2013). The restoration of rusted firearms: An evaluation of different methods. AFTE journal, 45 (3) 203-221

[19] Castro C, Norris S, Setume B, Hamby J. (2014). The examination, eval-uation, and identification of 40 S&W calibre cartridge cases fired from 1079 different Glock semiautomatic pistols manufactured over a six-year period. Canadian Society of Forensic Science Journal, 47 (2) 65-73 [20] Riva F, Champod C. (2014). Automatic comparison and evaluation of

impressions left by a firearm on fired cartridge cases. Journal of Forensic Sciences, 59 (3) 637-647

[21] Hamby J, Norris S, Petraco N. (2016). Evaluation of Glock 9 mm firing pin aperture shear mark individuality based on 1632 different pistols by traditional pattern matching and IBIS pattern recognition. Journal of forensic sciences, 61 (1) 170-176

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[22] Pollut M, Diaczuk P, Gambino C, Petraco N. (2016). Environmental effects on cartridge case primer shear marks. Forensic Science Journal (2016)

[23] Kirk J, Law E, Morris K. (2017). Estimation of changes in breech face and firing pin marks over consecutive discharges and its impact on an IBIS Heritage system. Forensic Science International, 278 (2017) 47-51 [24] Zhang K, Luo Y, Zhou P. (2017). Reproducibility of characteristic marks

on fired cartridge cases from five Chinese Norinco QSZ-92 9 x 19mm pis-tols. Forensic Science International, 278 (2017) 78-86

[25] Collender M, Doherty K, Stenton K. (2017). An investigation into the factors that influence toolmark identifications on ammunition discharged from semi-automatic pistols recovered from car fires. Science and justice, 57 (1) 41-52

[26] Brand N. (2017). Bewijskracht en correlatie van Glock slagpingatschaafs-poren. Final report HBO Forensic Science, Leeuwarden

[27] Zhang K, Luo Y. (2018). Slight variations of breech face marks and fir-ing pin impressions over 3070 consecutive firfir-ings evaluated by Evofinder. Forensic Science International, 283 (2018) 85-93

[28] Christen S, Jordi H. (2019). Individuality testing of Glock pistol barrel ”Marksman Barrel”. Forensic Science International, 295 (2019) 64-71 [29] Dong F, Zhao Y, Luo Y, Zhang W, Zhang K. (2019). Specificity of

charac-teristic marks on cartridge cases from 3070 consecutive firings of a Chinese Norinco QSZ-92 9 mm pistol. Journal of Forensic Science and Medicine, 5 (2), 87-94

[30] Addinall K, Zeng W, Bills P, Wilcock P, Blunt L. (2019). The effect of primer cap material on ballistic toolmark evidence. Forensic Science In-ternational 298 (2019), 149-156

[31] Manzalini V, Michele F, Casolari M, Causin V. (2019). The effect of com-position and morphological features on the striation of .22LR ammunition. Forensic Science International, 296 (2019) 9-14

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Appendix A: Table of all results

Part 1:

No. Author Title Year Goal of the study/experiment

Manual/aut omatic

Brand and model of firearm(s)/barrels Brand and type of ammunition

1 S. Kirby Comparison of 900 consecutively fired bullets and cartridge cases from a 455 caliber S&W revolver

1958 Repeatability Manual 1 455 caliber Smith and Wesson revolver 455 Colt “Super Clean” Smokeless (lead based)

2 D. Brundage The identification of consecutively rifled gun barrels

1998 Individuality Manual 1 Ruger P-85 semiautomatic pistol, with 10 consecutively rifled barrels

Winchester/Olin 9mm Luger

3 B. Bachrach A Statistical Validation of the

Individuality of Guns Using 3D Images of Bullets

2006 Part 1: Repeatability Part 2: Individuality Part 3: External factor: Damage

Automatic 1 unknown 9mm pistol with:

6 consecutively manufactured Taurus barrels, 11 consecutively manufactured Bryco barrels, 11 consecutively manufactured Beretta barrels, 11 consecutively manufactured HiPoint barrels, 11 Glock barrels,

11 S&W barrels,

12 consecutively manufactured SIG barrels, 15 consecutively manufactured Browning barrels, 11 consecutively manufactured Ruger barrels

Winchester 9mm 115g, Remington UMC 9mm 115g

4 T. Uchiyama Toolmark reproducibility on fired bullets and expended cartridge cases

2008 Repeatability Manual 1 Hi point C9 semiautomatic pistol Speer Gold Dot 115 9 mm Luger, Speer Gold Dot 124 9 mm Luger, Speer Gold Dot 147 9 mm Luger, DFA Frangible 9mm CT, Remington 9mm Luger

5 J. Hamby et al. The Identification of Bullets Fired from 10 Consecutively Rifled 9mm Ruger Pistol Barrels: A Research Project Involving 507 Participants from 20 Countries

2009 Individuality Both 1 Ruger P-85 9mm Luger caliber semiautomatic pistol, with 10 consecutively rifled barrels

Winchester 9mm caliber NATO

6 A. Sarıbey and A. Hannam An investigation into whether or not the class and individual characteristics of five Turkish manufactured pistols change during extensive firing

2009 Repeatability Both 1 9mm Parabellum Canik 55 1 9mm Parabellum Kanuni 16

1 9mm Parabellum Sarsılmaz Kılınç 2000 1 9mm Parabellum Yasuv 16

1 9mm Parabellum Sahin 08

MKE Parabellum 9mm

7 T. Grom and W. Demuth IBIS correlation results of cartridge cases collected over the course of 500 repeated firings from a glock pistol

2012 Repeatability Automatic 1 Glock model 23 40 Smith & Wesson caliber semiautomatic pistol

Winchester Target/Range, 40 Smith & Wesson cal 180g

8 D. LaPorte An empirical and validation study of breechface marks on .380 ACP caliber cartridge cases fired from ten consecutively finished Hi-Point model C9 pistols

2011 Individuality Manual 1 Hi-Point C9 9mm Luger caliber semiautomatic pistol with 10 consecutively made slides

Federal .380 automatic 95g with brass primer,

UMC .380 automatic 95g with nickel primer,

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9 T. Fadul Jr. An empirical study to evaluate the repeatability and uniqueness of striations/impressions imparted on consecutively manufactured Glock EBIS gun barrels

2011 Individuality Unspecified 1 9mm Glock pistol, with 10 consecutively manufactured Glock EBIS barrels

Federal 9mm

10 T. Weller et al. Confocal microscopy analysis of breech face marks on fired cartridge cases from 10 consecutively manufactured pistol slides

2012 Individuality Automatic 1 Ruger P-95DC 9-mm Luger caliber pistol, with 10 consecutively manufactured slides

Winchester 9mm caliber 147g

11 L. Chumbley et al. Clarity of microstamped identifiers as a function of primer hardness and type of firearm action

2012 Repeatability Manual 1 Sig Sauer P26 9mm semiautomatic pistol, 1 Taurus PT609 9mm semiautomatic pistol, 1 Hi-point C9 9mm semiautomatic pistol

Brown Bear 9mm 115g, DAG 9mm 124g,

Federal American Eagle 9mm 115g, Remington UMC 9mm 115g, PMC 9mm 115g, Silver Bear 9mm 115g, CCI Blazer 9mm 115g, Cor-Bon 9mm 147g, Independence 9mm 115g, Sellier & Bellot 9mm 115g

12 D. Mikko et al. Reproducibility of toolmarks on 20000 bullets fired through an M240 machine gun barrel

2012 Repeatability Manual 1 M240B machine gun 7.62x51mm M80 ball 150g

13 D. Mikko and J. Miller An empirical study/validation test pertaining to the reproducibility and of toolmarks on 20000 bullets fired through an M240 machine gun barrel

2013 Individuality/ Repeatability

Manual 2 M240B machine gun, with one extra spare barrel

7.62x51mm M80 ball 150g

14 T. Fadul Jr. et al An empirical study to improve the scientific foundation of forensic firearm and tool mark identification utilizing 10 consecutively manufactured slides

Unspecified 1 unspecified semiautomatic pistol, with 10 consecutively manufactured Ruger slides

Unspecified brand 9mm

15 T. Fadul Jr. et al. An Empirical Study To Improve The Scientific Foundation Of Forensic Firearm And Tool Mark Identification Utilizing Consecutively Manufactured Glock EBIS Barrels With The Same EBIS Pattern

2013 Individuality Manual 1 unspecified Glock pistol, with 10 consecutively manufactured EBIS barrels

Federal 9mm

16 C. Wong The inter-comparison of 1000 consecutively-fired 9mm Luger bullets and cartridge cases from a Ruger P89 pistol utilizing both pattern matching and quantitative consecutive matching striae as criteria for identification

2013 Repeatability Manual 1 Ruger P89 9mm Luger caliber semiautomatic pistol

Winchester/Olin 9mm 115g

17 A. Sarıbey and A. Hannam Comparison of the class and individual characteristics of Turkish 7.65 mm Browning/.32 automatic caliber self-loading pistols with consecutive

2013 Individuality Manual 10 Kırıkkale self-loading 7.65mm Browning/.32 Automatic caliber pistol with consecutive serial number,

10 Fatih 13 self-loading 7.65mm Browning/.32

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18 D. Mears The restoration of rusted firearms: An evaluation of different methods

2013 Repeatability/ External factor: Water, rust and rust removal methods

Manual 1 Tanfoglio Witness 45, 1 Star Firestar Plus 9mm, 1 Hi-Point CF380, 2 Ruger P89 9mm, 1 Beretta 96 40 cal, 1 Keltec P-11 9mm, 1 Ruger P95DC 9mm, 1 Kahr K40 40 cal, 1 Taurus PT92C 9mm American Eagle 9mm, American Eagle 40 cal, American Eagle 380

19 C. Castro et al. The examination, evaluation, and identification of 40 S&W calibre cartridge cases fired from 1079 different Glock semiautomatic pistols manufactured over a six-year period

2014 Individuality Manual 1079 Glock 40 S&W semiautomatic pistols S&W 40 calibre

20 F. Riva and C. Champod Automatic comparison and evaluation of impressions left by a firearm on fired cartridge cases

Automatic 15 SIG Sauer P228, 43 SIG Sauer P226, 23 SIG Sauer Sig Pro

Geco Sintox 9 mm Luger

21 J. Hamby et al. Evaluation of Glock 9 mm firing pin aperture shear mark individuality based on 1632 different pistols by traditional pattern matching and IBIS pattern recognition

2016 Individuality Both 1632 Glock 9mm semiautomatic pistols, including 12 with consecutively manufactured slides

9mm, brand not specified

22 S. Pollut et al. Environmental effects on cartridge case primer shear marks

2016 Repeatability/ External factor: exposure to moist environment

Both 1 Glock 19 9mm semiautomatic pistol Remington 9mm 115g

23 J. Kirk et al. Estimation of changes in breech face and firing pin marks over consecutive discharges and its impact on an IBIS Heritage system

Automatic 24 Ruger SR9 centerfire pistols Remington 9mm Luger

24 K. Zhang et al. Reproducibility of characteristic marks on fired cartridge cases from five Chinese Norinco SQZ-92 9x19mm pistols

Automatic 5 Norinco SQZ-92 9x19mm pistols Norinco DAP92-9

25 M. Collender et al. An investigation into the factors that influence toolmark identifications on ammunition discharged from

semi-automatic pistols recovered from car fires

2017 Repeatability/ External factor: Exposure to extreme temperatures

Manual 3 CZ-85B semiautomatic pistols Sellier and Bellot 115g with brass primer, CCI 115g with nickel primer

26 N. Brand Bewijskracht en correlatie van Glock slagpingatschaafsporen

Automatic 200 Glock 9mm semiautomatic pistols Fiochi 9mm parabellum with nickel primer S&W 9mm parabellum with brass primer

27 K. Zhang and Y. Luo Slight variations of breech face marks and firing pin impressions over 3070

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28 S. Christen and H. Jordi Individuality testing of new Glock pistol barrel “Marksman Barrel”

Automatic 10 Glock gen 5 17 semiautomatic pistols, 9 Glock gen 5 19 semiautomatic pistols

Ruag Action 4, Thun 14

29 F. Dong et al. Specificity of characteristic marks on cartridge cases from 3070 consecutive firings of a Chinese Norinco SQZ-92 9mm pistol

2019 Individuality Automatic 89 Norinco SQZ-92 9x19mm pistols Norinco DAP92 9mm

30 K. Addinnal et al. The effect of primer cap material on ballistic toolmark evidence

2019 Repeatability Automatic 1 Remington 7.62 700 “tactical” pistol Winchester 308 win with nickel primer, Wolf 308 win with brass primer, S&B 308 win with brass primer, PPU 308 win with brass primer, Patrone 308 win with brass primer, Hornady 308 win with brass primer

31 V. Manzalini et al. The effect of composition and morphological features on the striation of .22LR ammunition

2019 Repeatability Both 1 Tanfoglio Force 22L handgun CCI Blazer .22LR, Fiocchi TT Sport .22LR

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Part 2:

No. # of shots fired # of examiners

Trace analyzed # of

bullets/cartridge cases analyzed

Results Comments

1 900 1 Breech face marks, firing pin impressions and land marks

First five and every fifth afterwards

The cartridge cases showed little to no change. The marks on the bullets began changing after about 30 shots and became unmatchable after the 50th shot.

The author names the buildup of lead in the barrel as the primary reason for the changes.

2 120 per barrel 30 Land marks 450 No errors, 2 bullets were deemed inconclusive

3 Part 1: 220 bullets per barrel, one barrel per brand

Part 2: 10 per barrel per ammunition type, 10 barrels per brand Part 3: 2 per barrel per ammunition type, 10 barrels per brand. Purposefully damaged.

1 Land marks Part 1: 880 Part 2: 1700 Part 3: 340

Part 1: Ruger and Beretta barrels showed changes in the first few bullets fired. Browning and S&W barrels did not. For all barrels of this part there were minor changes over repeated firings, however none were significant enough to prevent a correct identification. Part 2: Ruger, Beretta and S&W barrels could easily be identified through this system. Taurus and Browning barrels could be identified with a bit more difficulty, and SIG, HiPoint and Bryco showed great difficulty. Winchester ammunition performed better than Remington.

Part 3: For moderately damaged bullets, the system performed significantly worse. Damage increases both false positive and false negative identifications. On average, the same brands that performed well in the previous part performed better here as well.

Part 2: The system had difficulty both with very cheap (HiPoint) and very expensive (SIG) guns. The cheap guns had difficulty because of the barrels being a bit wider, which left striations that were less deep and more inconsistent. The SIG barrels left very fine striations that the system also had trouble with. The author conjectures that bullet diameter and hardness are the main reasons for the differences in ammunition.

4 100, 20 per ammunition type

1 Breech face marks, firing pin

impressions, land marks

100 There were significant differences between the ammunition types, mainly in land marks and firing pin impressions. Identification of land marks between ammunition types was generally difficult. Striations on the bullets became less clear over the 100 shots, especially near the trailing edges of the marks. Cartridge cases remained comparable throughout the shots.

The author names weight, diameter and velocity of the bullets as possible influences on the

reproducibility of the land marks.

5 1680 per barrel 507 Land marks 7605 No errors, 8 bullets were deemed inconclusive This is a follow up study to D. Brundage (1998).

6 Canik 55: 1000 Kanuni 16: 2000 Sarsılmaz Kılınç 2000: 2500 Yasuv 16: 3500 Sahin 08: 5000

2 Breech face marks, firing pin impression, ejector marks, extractor marks,

For each pistol the first 10 and every 250th afterwards

Although minor changes were shown in both class characteristics and individual characteristics, there were no significant differences between the first and last fired cartridge cases.

The largest change over multiple consecutive shots was found in the extractor mark length of the Canik 55, but even this was not deemed significant.

7 500 2 Breech face marks, firing pin impression

The first, tenth, 25th, and every 50th afterward

The system returned the correct cartridge case in the top 20 for all cases for the breech face marks, and all but two of the firing pin impressions.

The IBIS BrassTRAX-3D system compared the cartridge cases to 909 other cases with similar class characteristics. The authors considers something a hit if the correct result appeared in the first 20 results.

8 200, 5 per ammunition type per slide

1 Breech face marks 200 All cartridge cases from the same slide could be identified with each other. On cartridge cases between slides there was limited agreement, however there was no significant agreement that could lead to a false conclusion.

The author called all types of ammunition as being marked equally well.

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10 9 per barrel 5 Breech face marks 90 The cross-correlation scores for matches (within distribution) and non matches (between distribution) are well separated. Subclass toolmarks were not found to have a mathematically detectable influence on the scores.

11 100 per ammunition type per gun

2 Firing pin microstamps

3000 The Sig Sauer performed the best, followed by the Taurus and finally the Hi-Point. There were differences with the ammo types, with the Brown Bear, UMC, and DAG being the best. No trends could be found for the analyzed variables, those including primer hardness score, primer material, and cartridge material. The only strong influence was whether the primers were lacquered, in which cause identifying the microstamps was almost impossible.

The Hi-Point was the cheapest of these guns, and the only one that used a blowback mechanism as opposed to a short recoil action for ejecting cartridges.

12 20000 3 Land marks 127 Significant differences were starting to appear after 10300 shots, however all bullets could still be matched with the first bullet.

Major marks that were very useful in the first few comparisons became less clear over time, causing the researchers to rely on other marks.

13 20000 for gun 1, 30 per barrel for gun 2

4 Land marks 164 All bullets could be matched with the barrel they came from, with an error rate of zero.

This is a validation study that combines the bullets from D. Mikko et al. (2012) with a new set of 60 bullets shot through a similar model of machine gun.

14 1000 per slide, for 5 of the slides 100 extra

217 Cartridge casings, otherwise unspecified

3500 The error rate of matching bullets with the slide they came from was found to not be significantly different from zero, with only one incorrect result and five inconclusive results. There were no significant differences in performance between examiners with more than ten years of experience and those with less. Bullets could still be matched with the slides even after a great amount of shots.

A lot of details in this study were really vague.

15 750 per barrel 165 Land marks 1650 A calculation based on a 95% confidence interval showed the error rate to be larger than 0 but smaller than 1.2%. There were no significant differences in performance between examiners with more than ten years of experience and those with less.

16 1000 1 Firing pin aperture shear marks, land marks

The first 10 and every 25th after

Each bullet could be successfully identified to the first bullet, and every aperture shear mark but three (the 725th, 800th and 825th cartridge case) could be identified. The striae seemed to become less pronounced over time, especially near the trailing edge.

When using the Quantitative Consecutive Matching Striae method, the author notes that later bullets had more shorter runs, and earlier bullets had more longer runs.

17 10 per pistol 2 Breech face marks, firing pin impression, ejector marks, extractor marks

200 Between pistols of the same model, class characteristics like firing pin impression diameter were found to not be significantly different, but individual characteristics were.

18 6 per pistol 1 Breech face marks, land marks

The 30 before rusting were compared with the 30 after rusting

In almost all cases, the breech faces went through significant changes when comparing before and after the rusting and rust removal. Soda blasting was found to be the best method for rust removal when ignoring all other factors.

Firearms that gave better results before rusting generally gave better results after rusting as well.

19 2158 4 Firing pin aperture shear marks

1079 All bullets could be individualized to the other bullet from the same pistol, to the exclusion of all others.

20 60 from 1 SIG Sauer 228, 60 from 1 SIG Sauer 226, 1 from all others

2 Breech face marks, firing pin impression

199 There was a significant difference in the found within distribution and the between distribution. The within distributions were found to be significantly different for the two main firearms.

The authors suggest that because the within distributions are so different, within distributions should be recalculated for every case where it would be relevant.

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at least 3 shear mark found to be 0.0001%. The IBIS system could correctly state that no bullet matched any other bullet.

from the 1632 total cartridge cases. The rest was analyzed manually.

22 25 in groups of five that were buried for different periods of time

4 Firing pin aperture shear mark

25 Although there were minor changes in the tinier striation lines, especially near the bottom, even after 12 weeks of being buried, the cartridge cases could still be matched with the unburied variants from the same gun. All but one of the studied variables (height distribution skewness) did not change significantly.

According to the authors, the change in the only significantly changed variable is most likely due to chance (it was a test at 5% significance level)

23 200 per pistol 1 Breech face marks, firing pin impression

4800 There were significant differences between the probability density functions of bullets from the same gun and from different guns, however the degree of separation was different for each pistol.

There was no decreasing trend to be detected in the effectiveness of the system used. The groups consisting of bullets fired far away from each other showed a bit more spread in distribution, but that can be explained as those groups were smaller.

For each pistol the first 20 and one randomly from every 10 afterwards

The distributions of similarity score from matches and non matches were significantly different. Both firing pin impressions and breech face marks were found to change slightly over time, however the changes were not significant enough to lead to any wrong conclusions through the system. For breech face marks,when comparing the first shot to other shots, there were extreme differences in similarity score before evening out after about 10 shots.

A possible explanation that the authors provide for the changing breech face marks is that there might be a substance on the breech faces of new guns that influences the marks, that runs out after a few shots after which the true breech face marks will appear.

25 6 per ammunition type per pistol

3 Breech face marks, firing pin impression, firing pin aperture shear marks, firing pin drag marks, land marks

3 shots per pistol per ammunition types fired after burning the guns were compared with 3 shots per pistol per ammunition type fired before

The barrels of the guns were damaged to the point that entirely new land marks were appearing and so they could not be matched anymore. Cartridge cases could be matched, although the quality of the traces had decreased greatly. Pistols left in the footwell of the car performed better than pistols left on the back seats.

Cartridge cases could only be matched when taking a combination of all possible traces (breech face marks, firing pin impression, firing pin aperture shear marks and firing pin drag marks). Pistols on the back seat were exposed to higher temperatures.

26 2 per ammunition type per pistol

10 Firing pin aperture shear marks

800 It was shown that the likelihood ratio given by the system reasonably correlates with the LR given by human examiners. The false positive percentage of the automated system was on average 2.691%, with fiocchi type ammunition performing better (at 1.263%) then S&W (at 3.371%)

The author names primer hardness and the possibility of there being lacquer left on the S&W cartridges as a possible reason for the different false positive percentages

For each pistol the first 20 and one randomly from every 10 afterwards

Breech face marks and firing pin impressions were found to change over many consecutive shots, with shots close together being more similar. However, the variation over these marks never exceeded the variation between marks of different firearms, so the Evofinder system was still able to correctly identify each bullet.

This is a followup to K. Zhang et al. (2017) that looks into how the markings change over time.

28 1 pistol 504 times all others 4 times

1 Land marks Pistol 1: Bullet 1 and every 50th bullet afterwards Pistol 2-19: All 4 bullets

The system always returned the other three bullets from the same gun in the top three positions. For the repeated shots no

downward trend could be detected. so identification would still be possible after 500 shots.

All pistols in this test are equipped with a Glock Marksman Barrel, a new type of barrel made to increase precision when firing. Before this,

identification of Glock barrels was considered difficult due to polygonal rifling.

29 3070 for pistol 1, 3 each for pistol 2-89

5 Breech face marks, firing pin impression

For pistol 1 the first 20 and one randomly from every 10

For both breech face marks and firing pin impressions, the within distribution (for bullets coming from the same gun) was

significantly different than the between distribution (for bullets coming from different guns). Firing pin impressions gave a higher

This is a followup to K. Zhang et al. (2017) partially using the cartridge cases from that study. Pistol 1 in this study is pistol A in that study.

Factors that could influence the repeatability and individuality of marks in the comparison of firearms: A literature review