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Gunshot Residue and Cross Contamination: An Introdcutory Lesson
By Steven Howard
Police are happy to point out: “The suspect tested positive for GSR and
this proves he shot the victim.” To the first part I respond: “So the
hell what?” To the second part: “Maybe!”
What is gunshot residue (GSR)? The Trace Evidence Procedures Manual
defines GSR as “a particle with a spherical or molten appearance
(non-crystalline) containing the elements Pb, Ba, and Sb.” The reality
is that about 99 percent of the time, the police and people like me do
not really test for gunpowder residue. Instead, we test for primer
residue.
To understand GSR, we will first look at gunpowder. Gunpowder is a
combination of chemicals that, once ignited, burn at a predictable rate.
This burning produces expanding gases that create pressure and force
the bullet down the barrel and towards the target. In the beginning, the
only gunpowder was black powder. This is a combination of 75 percent
potassium nitrate (also known as saltpeter), 15 percent charcoal, and 10
percent su
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