Crime Library: Criminal Minds and Methods

The Murder of Daniel Williams

The Firearms Examination

"Firearms are simply tools and whenever they're used, they leave behind characteristic tool markings," says Doreen Hudson, a supervisory criminalist in charge of LAPD's Firearms Analysis Unit.

Doreen Hudson

Cut into the barrel of every modern firearm is a series of rotating grooves that force a bullet to spin as it travels down the barrel. Much like a football, a bullet travels farther and straighter if it's spinning during flight. The grooves cut into a barrel leave scratches on the surface of every bullet fired through it. And like fingerprints, the scratches are unique. No two are exactly the same. Under microscopic examination, a spent bullet can be matched to a single firearm by comparing it with a test bullet fired from the same gun.

Likewise, semiautomatic firearms, like the ones used in both the Newton and Hollenbeck transvestite murders, leave distinctive marks on the shell casings of each cartridge. The marks are cause by the firing pin, the thin metal rod that strikes the explosive primer centered in the back of the cartridge case; and by the extractor, an L-shaped piece of metal that catches the rim of the cartridge case and yanks it out of the chamber after the cartridge has been fired. The marks are unique, and, as in the case with spent bullets, under microscopic examination they can be matched to a specific firearm.

In the Newton and Hollenbeck murders, although investigators had not found a firearm, they requested that the Firearms Unit compare the bullets recovered from both bodies and the shell casings found at the two crime scenes to determine if they were fired from the same gun.


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