Extracorporeal bullet trajectory determination from scanned phantoms with bullet defects.

Shots with two different calibres (0.32 Auto and 9 mm Luger) were fired through phantoms that simulated human torsos, mounted on undercarriages with witness panels. The perforated phantoms were scanned with computed tomography (Siemens) using 80 kV and 140 kV and a slice thickness of 1 mm. The intracorporeal trajectories in the phantoms were compared to the known extracorporeal trajectories, derived from the perforations in witness panels. The discrepancy between the intracorporeal and extracorporeal trajectories, denoted as the absolute angle, was calculated for the trajectories before (front) and after (rear) the phantoms. Mean absolute angles at the front were lower than at the rear (2.27° vs. 4.54°) and the difference was statistically significant (p < 0.001). The results of the study imply that the line between the entrance and the exit wound in a scanned victim can be extended to the extracorporeal bullet trajectory leading towards the entrance wound. The absolute angles presented in this study give an impression of the expected errors with the two calibres. This can be helpful in shooting investigations to assess the position of the shooter from entrance and exit wounds in a scanned victim.

The influence of impact velocity on bullet trajectory deflection through ballistic gelatine.

This paper presents the results of a study on bullet trajectory deflection, for 9 mm Luger Full Metal Jacket Round Nose (FMJ-RN) bullets fired through 23-24 cm of ballistic gelatine. The bullets were fired at different velocities. Impact velocity, energy transfer and bullet trajectory deflection after gelatine perforation were measured and calculated. As was expected, energy transfer to the gelatine blocks generally increased with increasing impact velocity, indicating an altering bullet/gelatine interaction with altering velocity. This alteration did not result in a discernible alteration of bullet trajectory deflection. Deflection angles fell between 5.7° and 7.4° for 136 of the 140 fired shots, with four outliers below 5.7°.

A combined cowhide/gelatine soft tissue simulant for ballistic studies.

The ballistic resistance of a combined soft tissue simulant was studied, consisting of gelatine as a simulant for human muscle tissue and tanned cowhide (leather) as a simulant for human skin. The simulant was manufactured by applying cowhide to liquid ballistic gelatine, as the gelatine solidified in its mould. Combining a skin and muscle tissue simulant in this adhered way opens the possibility to produce purpose-built proxies for human body parts in ballistic studies or for forensic shooting incident reconstructions. Ballistic resistance of adhered cowhide - (bonded) to solidifying ballistic gelatine - was compared to that of the same material applied on gelatine blocks in loose condition. Ballistic resistance of tanned cowhide was found to be more consistent in adhered condition. This enhanced consistency is a benefit, increasing reproducibility of results in ballistic studies. Additionally, two ways to assess ballistic resistance of a skin simulant were described and compared. Logistic regression, from a number of measured velocities and associated (non)perforations is recommended for testing ballistic resistance.

Error estimation on extracorporeal trajectory determination from body scans.

This study explores the magnitude of two sources of error that are introduced when extracorporeal bullet trajectories are based on post-mortem computed tomography (PMCT) and/or surface scanning of a body. The first source of error is caused by an altered gravitational pull on soft tissue, which is introduced when a body is scanned in another position than it had when hit. The second source of error is introduced when scanned images are translated into a virtual representation of the victim's body. To study the combined magnitude of these errors, virtual shooting trajectories with known vertical angles through five "victims" (live test persons) were simulated. The positions of the simulated wounds on the bodies were marked, with the victims in upright positions. Next, the victims were scanned in supine position, using 3D surface scanning, similar to a body's position when scanned during a PMCT. Seven experts, used to working with 3D data, were asked to determine the bullet trajectories based on the virtual representations of the bodies. The errors between the known and determined trajectories were analysed and discussed. The results of this study give a feel for the magnitude of the introduced errors and can be used to reconstruct actual shooting incidents using PMCT data.

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.

Influence of bullet type and muzzle-to-target distance on trajectory deflection through a soft tissue simulant.

The influence of two different variables on deflection was studied with 9mm Luger calibre bullets, fired through ballistic gelatine. To test whether bullet type affects trajectory deflection, bullets of four different types were fired (20 shots per type) through 25cm of 10% gelatine at a 5m muzzle-to target-distance. A Kruskal-Wallis test showed that the deflection of at least one bullet type was significantly different (p<.001, significance level of 0.05) from the deflection of the other bullet types. To test whether muzzle-to target-distance affects trajectory deflection, additional shots were fired with one of the four bullet types (7.5g FMJ-RN) at 1 and 15m muzzle-to target distances (20 shots per distance), again through 25 cm of 10% gelatine. A Kruskal-Wallis test showed that there was no significant difference (p=.72, significance level of 0.05) in deflection between the three muzzle-to target distances of 1, 5 and 15m.

Relationship between bullet diameter and bullet defect diameter in human calvariums.

Existing literature on the relationship between bullet diameter and bullet defect diameter in the human calvarium is summarized and discussed. The hypothesis, derived from the literature, that bullet deformation influences bullet defect diameter was studied in a small controlled experiment. The mean defect size caused by non-deforming projectiles was found to be smaller than the mean defect size caused by deforming projectiles of equal original mass and size. The p value of the difference between the two means, measured in two different ways, was found to be 0.002 for both in a Mann-Whitney U test and was significant if the confidence level is set at 5%.

Rifle bullet deflection through a soft tissue simulant.

Trajectory deflections of 5.56 NATO and 7.62×39mm rifle bullets, fired through 7.5, 15 and 22.5cm of gelatine, were studied. The magnitude of the deflections from the bullets' original trajectories after perforation are related to the length and the profile of the (wound) channels through gelatine. After 7.5cm of penetration depth, deflection was less than 1°. With the longer channel lengths, bullet instability set in and subsequently, deflection was much larger. Deflection was highest with fragmented 5.56 NATO bullets after perforating 22.5cm of gelatine. The data from this study can be used to assess the degree of bullet deflection in trajectory reconstructions after incidents where human bodies were perforated with rifle bullets of the respective calibres and cartridge types.

Pistol bullet deflection through soft tissue simulants.

Trajectory deflections of pistol bullets from four different firearms, fired through soft tissue simulants under two different incidence and exit angles were studied. The data from this study can be used in reconstructions of shooting incidents where human soft tissues (not bones) were perforated with pistol bullets and assumptions must be made about bullet deflection in order to correctly reconstruct trajectories. The results demonstrate that deflection was influenced by the length of the "wound channel" through the simulants. In short, deflection was small to virtually absent with 5 and 10cm channel lengths. With channel lengths of 15, 20 and 25cm, there was a clear increase in deflection and/or a more erratic deflection behaviour with most shots. The data also suggest an influence of the angle of incidence and/or exit on both the direction and the magnitude of the deflection.

A part-declared blind testing program in firearms examination.

In 2015 and 2016 the Central Unit of the Dutch National Police created and submitted 21 cartridge case comparison tests as real cases to the Netherlands Forensic Institute (NFI), under supervision of the University of Twente (UT). A total of 53 conclusions were drawn in these 21 tests. For 31 conclusions the underlying ground truth was "positive", in the sense that it addressed a cluster of cartridge cases that was fired from the same firearm. For 22 conclusions the ground truth was "negative", in the sense that the cartridge cases were fired from different firearms. In none of the conclusions, resulting from examinations under casework conditions, misleading evidence was reported. All conclusions supported the hypothesis reflecting the ground truth. This article discusses the design and results of the tests in more detail.

Implementing context information management in forensic casework: Minimizing contextual bias in firearms examination.

Managing context information in forensic casework aims to minimize task-irrelevant information while maximizing the task-relevant information that reaches the examiner. A design and implementation of context information management (CIM) is described for forensic firearms examination. Guided by a taxonomy of different sources of context information, a flow-chart was constructed that specifies the process of casework examination and context information management. Due to the risk of bias, another examiner may need to be involved when context information management is unsuccessful. Application of context information management does not make a subjective examination objective, but can limit the risks of bias with a minimal investment of time and resources.

Design and results of an exploratory double blind testing program in firearms examination.

In 2010, the Netherlands Forensic Institute (NFI) and the University of Amsterdam (UvA) started a series of tests for the NFI's Firearms Section. Ten cartridge case and bullet comparison tests were submitted by various external parties as regular cases and mixed in the flow of real cases. The results of the tests were evaluated with the VU University Amsterdam (VUA). A total of twenty-nine conclusions were drawn in the ten tests. For nineteen conclusions the submitted cartridge cases or bullets were either fired from the questioned firearm or from one and the same firearm, in tests where no firearm was submitted. For ten conclusions the submitted cartridge cases or bullets were either fired from another firearm than the submitted one or from several firearms, in tests where no firearm was submitted. In none of the conclusions misleading evidence was reported, in the sense that all conclusions supported the true hypothesis. This article discusses the design considerations of the program, contains details of the tests, and describes the various ways the test results were and could be analyzed.