Solving a case like the October sniper shootings around Washington, D.C., hinges heavily on firearms identification. How do examiners match evidence to a gun? If a gun is recovered, a forensic scientist test-fires it to determine the markings it leaves on bullets and cartridge casings. The examiner then compares these under a microscope with the caliber (diameter), rifling pattern (series of grooves), and impressions and striations (microscopic marks left by unique imperfections in a gun's firing pin and barrel) on the bullets and casings found at the crime scene.

If test-fires don't match, or if no gun is found, the examiner will measure the rifling patterns on the recovered ammunition and compare them with the General Rifling Characteristics database to see which gun models might match. The problem in some cases, however, is that "20 to 150 brands of firearms could leave the same rifling pattern," says Scott Doyle, forensic specialist for the Kentucky State Police in Louisville.

This article was originally published with the title Scratch Match.

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