Introduction
Wound ballistics investigates the interaction of projectile and tissue. The injuring capacity of a bullet arises from its kinetic energy. If the energy density of a bullet is larger than the tissue specific value, the bullet penetrates [
1]. Such a non-elastic impact of a bullet on a biological target leads to the tissue being crushed [
2,
3]. Crushed tissue is then dispersed within the wound channel [
4]. In real gunshot injuries the penetration of a bullet causes multiple vessel lacerations with subsequent bleeding, primarily inside the wounded area [
2], together with external bleeding at the entry and exit wounds. In summary, a perforating projectile can cause crushed tissue and bloody fluid to accelerate into the direction of fire (“forward spatter” [
5,
6]) or against the direction of fire (“backspatter” [
7,
8]). As expected, forward spatter is associated with the exit wound and backspatter with the entry wound. Backspatter can be observed in many (but not all) cases of gunshots to the head [
7,
8].
During the experimental investigation of staining inside firearm barrels [
9], transparent target models (“reference cube” [
10]) were introduced. The reference cube was equipped with a thin foil bag containing acrylic paint, barium sulfate and blood, which was mounted beneath the front cover made by an absorbent kitchen wipe. Although the reference cube was originally designed to create optically visible traces in gun barrels, it was also possible to observe the propagation of colored material in and against the direction of fire. At the same time, high-speed video (HSV) showed the passage of the projectile and the formation of the temporary cavity (TC). HSV enables measurements of the bullet velocity [
11] and its deceleration [
12]. Previous studies illustrated the additional influence of muzzle gases on the TC in contact [
13] or near contact shots [
14]. The observed properties of the target model can contribute to a more comprehensive understanding of “backspatter”.
The present study investigates back spattered fluid in the context of experimental gunshots to the 12 x 12x12 cm3 gelatin reference cube.
Discussion
In general a bullet is decelerated whilst passing through a soft target medium. The kinetic energy lost is transferred to the target medium. In transparent gelatin models, it is possible to observe this process using high-speed cameras [
12]. Gelatin is radially displaced forming a temporary cavity (TC), which collapses a few milliseconds later. Due to the elastic property of gelatin, it goes on to form another, smaller TC, which then takes longer to collapse and so on (Fig.
3). Summarizing, the gelatin movement in all directions corresponds to a decreasing oscillation until all deposited energy is consumed. In order to simulate contact shots in gelatin, the blocks had to be covered on the entry side to force the muzzle gases to enter into the block. This was achieved using a "reference cube" [
10]. This 12-cm long head surrogate is covered by an absorbent kitchen wipe beneath which a flat reservoir of viscous acrylic paint is fixed. High-speed video (HSV) of shots to reference cubes showed not only the bullet-"tissue" interaction with the expanding and collapsing TC, but also what happened to the fluid sealed in the paint pad.
Astonishingly, at the moment when the bullet penetrated the model and perforated the paint pad, retrograde ejection of paint was never observed. In distant shots, only a short crown-like protrusion with back spattering surface material was observed for approximatively 0.2 ms beginning with the bullet penetration (Fig.
1). This result corresponds to the findings of Radford et al. [
15] who shot live anaesthetized pigs using 9 mm Luger FMJ. Following their definition, this phenomenon might be interpreted as tail splashing (crushed material back streaming over the projectile). Black et al. described larger tail splashing caused by the penetration of the projectile, when they shot at bare gelatin blocks [
16] (Online Resource
5). This confirms the importance of the tight cover used in the reference cube model [
10]. Further, Radford et al. described a ballooning of the pig skin for 0.7 ms [
15] which exactly matches the time of the ballooning of the front cover in the reference cube model.
All shots (n = 102) caused a retrograde ejection of liquid, but at a different point in time. The results indicate a marked difference between shots from distance (≥ 5 cm, n = 51) and (near) contact shots. This difference concerns the moment as well as the type and the velocity of liquid ejection.
Distant shots, independent of the caliber, provoked a linear jet, which started in the late TC's collapse phase or after its collapse. The deviation of the jet from straight backward (0°) was moderate and stochastically distributed. The backward travelling end of the jet was often curved like a tongue (Fig.
5,
6A,
6B). Once the jet had started, it continued in a band-like stream, sometimes exhibiting a slow twist (Online Resource
3). Finally the jet got thinner and broke off after about 60 to 100 ms, when the visible movement of the gelatin cube had stopped. In some shots, a bulb- or tulip-like widening was observed in the chronological context of the second TC's collapse. The velocity of the jet varied widely (6 – 45 m/s) and is not related to ammunition or shooting distance. Comiskey et al. published similar data derived from videos for blood pattern analysis using bare and covered sponge models [
17,
18], although the image quality of their videos was inferior. The analysis of high-speed films of distant shots (9 mm Luger) to living calves resulted in initial blood drop velocities of 13 to 61 m/s [
19]. In the Handbook of forensic medicine, Karger gives the order of 10 m/s [
20]. Lazarjan et al. measured 21 to 37 m/s for the ejection of brain material after distant shots to slaughtered ovine and bovine heads [
21].
In an article on backspatter published in 1931, Weimann mentioned the observation of hunters concerning a light reflex when the game was hit by the bullet [
22]. It was interpreted as a blood stream from the entry wound. Considering the results obtained using handguns, it might be possible that a jet of blood caused by hunting ammunition could be perceived by the eyes.
When the muzzle to target distance decreased to 3 cm and less, the character of liquid ejection changed significantly. Aerosol-like substance escape as well as cone-like spray or spatter were observed. Mainly, the release of liquid was not continuous and occurred in several phases with higher velocities (up to 330 m/s). In contrast to distant shots, the result depended on ammunition and distance. At close range, it was obvious that muzzle gases were blown into the target model. The expansion of the TC was larger [
14] and its collapse took longer. In contrast to distant shots, where a rhythmic undulation of the TC was observed, close range shots showed irregular movement after the first collapse of the TC. The hypothesis that muzzle gas might be trapped inside the gelatin, preventing a complete collapse of the TC, [
14] was vividly confirmed by one or several gaseous eruptions, which followed the first collapse (Fig.
7). The results reflected the increasing influence of muzzle gas [
14], which is determined by the cartridge, the barrel length [
23,
24] and the gap between muzzle and target. The variable of the barrel length was eliminated by using only four inch barreled firearms in accordance with previously published articles [e.g. 13]. The influence of energy transfer was reduced by choosing exclusively non-deforming full metal jacketed bullets.
The study confirmed the results published by Wagner [
25] who shot anaesthetized rabbits using a 7.65 mm Browning (.32 auto) from various distances up to contact. He reported a significant decrease of backspatter when the distance was 2.5 cm or more. The knowledge of muzzle gas effects that depend on cartridge load and distance in close range shots was already documented in the handbook of Hofmann 1881 [
26]. Hofmann also mentioned experimental close range and contact shots to cadavers, which provoked backspatter of powder and tissue debris on the shooter's hand. Since then many forensic pathologists have investigated back spattered material on the hands, on the weapon, and inside the barrel after contact shots [e.g. 22, 27–29]. Experimental research used either biological targets (living rabbits [
25], living calves [
19], living pig and pig heads [
15], human cadavers [
30], ovine and bovine heads [
21]) or blood soaked sponges [
5,
31‐
33]. All these approaches have particular advantages, but the common disadvantage was that the targets are not transparent and are dissimilar to human heads. The investigation of staining inside firearm barrels initiated using non-transparent silicone covered hemispheres [
34], boxes [
24] or polyethylene bottles [
35]. With introduction of the "reference cube" with 12 cm edge lengths and a weight of 1.7 kg [
10], a cheap transparent gelatin target model was available for reproducible ballistic experiments without ethical issues. Even though the "reference cube" is dissimilar from a vital human head, this surrogate has allowed the study of staining inside firearm barrels [
13] as well as wound ballistic effects [
14,
36] and their influence on "backspatter".
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