Comparison and interpretation of impressed marks left by a firearm on cartridge cases - Towards an operational implementation of a likelihood ratio based technique.

Firearm examination is subject to increased scrutiny regarding its foundational validity and inherent subjective nature. The increased use of automatic comparison systems may help to reduce subjectivity. In this paper, we present the performance and limits of an automatic comparison system that assigns a weight to the forensic findings for the comparisons between firing pin marks, breechface marks, or a combination of the two. This weight is expressed by a likelihood ratio (LR) based on 3D topographical measurements coupled with a bi-dimensional statistical model. As the performance of such systems may depend on the reference databases used to inform the model, we investigated the impact of the brand of ammunition and the number of samples. We show that reference databases used to calculate LRs should ideally consist of the same type of ammunition as is seen in the case under investigation and that 7 specimens fired by the same firearm are enough to obtain rates of misleading evidence of a similar magnitude compared to those obtained when far more specimens (60) are used. Additionally, the automatic system was used to assess the outcomes of 7 cases with known same-source or different-source ground truths. These cases were also examined by 8 qualified firearm examiners. In all cases, the experts' appraisals were in line with the ground truth. The automatic system showed some limitations in cases were the data were not sufficient to calculate a robust LR, but also that it can assist and enhance the examiners in their decision process.

Objective Evaluation of Subclass Characteristics on Breech Face Marks.

Subclass characteristics can be found on the breech face marks left on spent cartridge cases. Even if they are assumed to be rare and their reported number is small, they can potentially lead to false associations. Subclass characteristics have been studied empirically allowing examiners to recognize them and to understand in which conditions they are produced. Until now, however, their influence on the identification process has not been studied from a probabilistic point of view. In this study, we aim at measuring the effect of these features on the strength of association derived from examinations involving subclass characteristics. The study takes advantage of a 3D automatic comparison system allowing the calculation of likelihood ratios (LRs). The similarities between cartridge case specimens fired by thirteen S&W .40S&W Sigma pistols are quantified, and their respective LRs are computed. The results show that the influence of subclass characteristics on the LRs is limited, even when these features are prevalent among the potential sources considered in a case. We show that the proportion of firearms sharing subclass characteristics should be larger than 40% of the pool of potential firearms for the effect to be significant.

Interaction of bullets with intermediate targets: material transfer and damage.

In complex shooting incidents, it is not always clear which bullet hit or eventually killed the victim and who fired it. The examination of traces of foreign material embedded in or adhered to bullets provides critical information in the trajectory reconstruction of spent bullets. Such a reconstruction can have considerable legal implications because it can prove that it was not someone's intention to kill. However, the microtraces that remain on spent bullets are often ignored. Microtraces on bullets, around bullet-holes and on ricochet marks were investigated using SEM/EDX for two different types of bullets: a relatively hard, full metal jacket (FMJ) bullet and a relatively soft, lead round-nose (LRN) bullet. A total of 179 bullets were fired into intermediate targets, sheets of 5 different materials (MDF, greenboard, gypsum fibreboard, glass and steel), at approximate incident angles of 90°, 10° and 5°. Of the 144 bullets fired at incident angles of 90°, 130 bullets perforated one of the materials, and 14 bullets perforated two of the materials. The 35 bullets fired at incident angles of 10° and 5° ricocheted off the intermediate targets, producing ricochet marks. In the majority of cases, traces from the target materials were found on the bullet, both after perforation and ricochet. The only exceptions were (1) the perforation of 9-mm sheets of MDF by FMJ bullets and (2) ricochet off glass when the glass did not break. Steel targets transfer small, but still detectable traces of iron to the bullet. The order in which targets are hit was reflected in the traces found on the bullets, i.e., materials from a secondary target were deposited on top of deposits from the primary target. This result implies that it is possible to determine the order of impact from the stratification of the material analysed. Traces from the bullets were found around all the bullet holes. Wipe-off from lead bullets is sometimes visible by the naked eye. Ricocheting bullets produce remarkable traces on glass. The combination of sliding and flattening action leaves a trace of the bullet's surface material in the form of tiny droplets and other evidence of liquid formation. In general, softer LRN bullets are more susceptible to the transfer of material than harder FMJ bullets. For perforation, trace materials are preferentially deposited in a ring-shaped area around the (flattened or unflattened) nose of the bullet.