The Champion

September/October 2007 , Page 38 

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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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