Crime Scene Photography by Professor Bob Warnock and Jerry Dambrogio. Gun Shot Residue Class.
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Gunshot residue (GSR), or more technically, gunshot primer residue, is expelled as tiny particles from the barrel of a firearm when it is fired. Among other materials, gunshot residue contains the heavy metals barium, lead and antimony. Modern forensic methods require the presence of these metals to identify trace evidence as gunshot residue.
Through the 1990s the presence of two or more of these heavy elements was sufficient to conclude the chemicals were the result of a gunshot. New, more rigorous standards for gunshot residue testing are now being required by many courts. The trend is toward a standard that requires the presence of at least one Pb-Ba-Sb particle (lead-barium-antimony) exhibiting characteristic GSR morphology and supported by a sufficient number of atomically identified GSR supporting particles in the remaining particle population.
Investigators may test a suspect's hands, arms and face for particles of gunshot residue as evidence of having recently handled or fired a gun. Current testing methods are inconclusive as a suspect's clothing or body can easily become contaminated after the firing from sources such as being handled by police officers or being transported in contaminated police vehicles