Factors That Could Influence the Repeatability And

Factors that could influence the repeatability and individuality of marks in the comparison of firearms: A literature review Master Forensic Science Supervisor Examiner Jorit Delen Martin Baiker Erwin Mattijssen 12345393 January 2020 Abstract Repeatability and individuality are important concepts in firearm comparison. 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. 1 Contents 1 Introduction 3 2 Research question 4 3 Search strategy 5 4 Results and discussion 5 4.1 Repeatability . 5 4.1.1 Repeated firing and wear . 6 4.1.2 Firearm brand . 8 4.1.3 Cartridge material and dimensions . 10 4.1.4 External effects . 11 4.2 Individuality . 12 4.2.1 Subclass characteristics . 12 4.2.2 Firearm brand . 13 4.2.3 Cartridge material and dimensions . 15 4.2.4 External effects . 16 5 Conclusion and recommendations 17 References 17 Appendix A: Table of results 21 2 1 Introduction Forensic firearm examination is a very important discipline within forensic science. 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 rifled, 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 3 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 cartridge 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 material 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: 4 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 repeatability 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. 5 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.

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