As the bullet passes down the barrel, the rifling will tear off small fragments of the bullet. Some of these fragments will remain in the bore and others will be blown out of the bore by the gases following the bullet. Some of these fragments do, however, remain attached to the bullet in the form of sub-microscopic pieces of swarf. As the bullet passes through any material, whether it is human flesh, fabric or wood, these fragments are often transferred to the medium through which it is passing.
These fragments are exceedingly small, but if an adhesive taping is taken from the periphery of the bullet entrance hole, they can be recovered. Examination under the electron microscope will enable these fragments to be qualitatively analyzed and the bullet type and/or country of origin identified.
It is important in the interpretation of these results to distinguish between volatilized lead from the bullet base and lead fragments from the bullet driving surface. Volatilized lead will be spheroidal or have smooth contours, whilst lead from the bullet 's surface will be more rough and swarf-like. The importance here is that as the metallic components of the lead alloy will volatilize at different rates, the results from the volatilized lead will be different from those torn off the driving surface.
Likewise, it is important to distinguish between copper zinc alloys from contamination and those particles torn from the bullet' s jacket by the rifling. Size and morphology is one identifier; the other is from the quantitative analysis, if this is possible.
Examples would include the identification of lead and copper/zinc alloy as coming from a semi- jacketed bullet. Another would be the identification of copper-coated steel from 7.62 x 25 mm ammunition of Chinese manufacture.
With larger fragments, accurate quantitative analysis will often enable the make of ammunition to be determined. The fragments recovered from the periphery of bullet entry holes are, however, invariably too small for this type of analysis. The problem here is that for accurate energy-dispersive X-ray (EDX) analysis via the scanning electron microscope (SEM), a mirror-like surface is required.
With larger fragments, 0.25 mm or greater, which can be seen under an optical microscope, it is possible to crush them between microscope slides to give the mirror-like surface required. The rough irregular swarf-like shape of the particles torn from the bullet's driving surface is normally too small for manipulation and is therefore unsuitable for this type of analysis.
Case example: Police exchange of fire. During a particularly bad shoot -out with a gang of armed robbers, five innocent bystanders were injured and two were killed by gunfire. The police were using semi -jacketed 0.38" Special+P calibre revolver ammunition and the robbers 7.62 x 25 mm Chinese ammunition. In the wound of one of the victims, there was a bullet fragment, and this proved to be a very small piece of copper/zinc jacket from a police round. From its appearance, this had obviously ricocheted before striking the bystander. The rest of the victims all had fully penetrating wounds.
It was obviously of some importance to show whether any of the other bystanders had been accidentally shot by the police. Tapings were taken from the entry holes, and an analysis of these revealed the presence of copper-coated steel fragments in all instances. This proved beyond reasonable doubt that they had all been shot by the robbers and not the police.
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