Monday, May 10, 2004

I got some emails regarding ballistics evidence in reponse to my previous blog on the LAT's story about the LAPD crime lab. One of the readers wanted to know if gun "fingerprinting" that he'd read about was as unique as human fingerprints. Another asked a more basic question about what exactly a crime tech does to identify a shell casing or spent projectile.

While a complete answer could fill a book, I'll address both as briefly as possible.

First off, a loaded round consists of four components: the projectile, the shell casing, the powder charge and the primer that ignites the powder. The projectile is found either in the victim, buried in a car, wall or whattever, and the powder is completely consumed in firing. What's left for analysis is the casing and the primer. The projectile is a separate discussion I'll leave for another time. Right now we're talking about shell casing analysis.

There are two basics types of handguns. Semi-autos and revolvers. When a semi-auto is fired, the slide ejects the spent round (shell casing) and automatically feeds a fresh round into the chamber. A revolver on the other hand, retains the spent shell casing in the cylinder. If a revolver is fired at a crime scene, chances are, there will be no spent shell casings on the ground. That is, unless it was a long firefight and the combatant(s) shot dry and then reloaded. This is highly unusual.

Semi-autos are used way more frequently and the spent shell casings are what's under all those paper tents you see on TV at crime scenes.

When a round is fired, the shell casing is subjected to a number of forces that leave scratch marks on its surface. The shell casing is generally made of brass, a material that's softer than the steel of the gun. Because it's softer, it's imprinted with the tool marks and other marks unique to the gun. Think of it as pushing modeling clay into a mold and lifting out the clay. What you have on the clay is the imprint of the mold.

After firing, the shell casing has the firing pin mark on the primer, the mark of the gun's extractor on the rim of the shell casing, scratch marks from the chamber of the gun and imprints of the gun's breechface on the base of the shell casing. These imprints and scratches are the result of the tool marks put on the gun during the manufacturing process. An analyst can determine with a high degree of accuracy if shell casings from different crimes all came from the same gun. The one caveat is that the gun has not been altered between crimes.

Unless you're dealing with a fairly sophisticated criminal, guns are rarely altered. However, it's not unheard of. I've sat in enough court trials where informants have admitted to receiving guns used in a crime and altered them before the next use.

How is that done? It's not that hard. The firing pin and extractor can be easily replaced. Even though these new parts are the same brand as the original, the tool marks are probably different enough to change the "fingerprint" of the firing pin and extractor. As to the chamber, which is part of the barrel, that too can be replaced or altered. Like the chamber, the breechface can also be altered with nothing more sophisticated that sandpaper and elbow grease. If you want to get fancy, you can by a $40 Dremel tool and a polishing stone attachment and completely change the "fingerprint" of the breechface.

So to answer the two readers, shell casings can be analyzed and traced back to a specific gun thanks to the toolmarks. If, that is, the gun hasn't been altered as I outlined. And this speaks directly to the second question about uniqueness of gun "prints." Gun fingerprints, unlike human prints, can change either through use of the gun (all that rubbing and pressure will in time have an effect on the toolmarks) or deliberately altered.

And that leads to my personal opinion on the proposed nationwide "printing" of all guns prior to sale. Lawmakers wanted to create a nationwide database of gun prints cross-referenced with the serial number of the gun and the name of the purchaser. It's ultimately an exercise that can be rendered pointless by the use of sandpaper and/or replacement parts. Gun prints are not permanent and that whole database will amount to nothing more than an elaborate and expensive collection of useless scratches.

No comments: