Trajectory reconstruction from trace evidence on spent bullets
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Contact of a fired bullet with an intermediate target of sufficient resistance commonly causes the bullet to ricochet, fragment or perforate together with a deviation in trajectory. The transfer of intermediate target material to bullets and subsequent detection on the bullet surface after recovery from a bullet collector, were examined using a scanning electron microscope and an energy-dispersive X-ray spectrometer (SEM/EDS). A total of 76 gunshots (9 mm Luger FMJ RN bullets) were fired at various intermediate targets and at combinations of intermediate targets and tissue located in line. Elements already present on unfired bullets and elements from the bullet collector, the jacket, the charge and primer could be consistently detected as a “background”. Abundant deposits of “fragile” (brittle) materials such as concrete, flat glass, asphalt and gypsum board could be visualised on every bullet by SEM. The transfer dynamics involved a direct imprint of target material on the bullet surface and thus preferential locations at the tip but also indirect deposition over the entire surface (“powder effect”). X-ray microanalysis demonstrated matching spectra of the elemental composition of these deposits and of the targets contacted. After perforation of “ductile” (flexible) materials such as wood and car body parts, the deposits on the bullets did not show characteristic spectra. If multi-layered car metal targets were hit, few and uncharacteristic fragments were scattered over the bullet surface and titanium indicative of paint-work could be determined on only a minority of bullets. The elemental composition of wood itself was heterogeneous but the fibrous morphology of the deposits was typical. The SEM/EDS findings in gunshots including subsequent perforation of tissue were similar. In particular, the trace evidence primarily transferred to the bullets was not eliminated by secondary contact and the determination of the fragile target materials was not affected. So when a person is killed or injured by a gunshot, the presence of a ricochet and the target material can be determined. This possibility needs to be considered before an evidential bullet is cleaned or contaminated.
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