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SHOT THROUGH THE HEART - What Gunshot Residue Can and Can't Tell

© Elizabeth Becka Lansky

I can assure you from personal experience that guns are still the weapons of choice for most American murders. Assuming one figures into your plot, what kind of gunshot residue evidence can be used against your villiain/hero/red herring?

Let’s start with how a gun works, for those of you who aren’t familiar with them. If you look at the back of any bullet larger than a .22, you’ll see a small circle in the center (known as ‘centerfire’). This is a short, sealed cylinder containing primer. When the trigger is pulled, the hammer of the gun hits this cylinder, and the shock of that blow causes the primer to explode. This explosion blows out into the large chamber of the casing, where the gunpowder is, causing the gunpowder to catch fire. The burning of the gunpowder is what expels the slug away from the casing, down the barrel of the gun and out the open end.

(Some small caliber bullets, like .22s, may have the primer crimped into the rim of the bullet instead of in a center cylinder, and the hammer can strike the bullet anywhere on the rim to fire the slug. This is known as ‘rimfire’.)

Tiny particles of this primer material (gunshot residue or GSR) are expelled from the front of the gun. They also ‘leak’ from the breechblock area when the gun is fired, either from the open back of the chamber in revolvers, or when chamber opens and the cartridge is ejected in automatics. This way, GSR can settle on the shooter’s hand.

Primer is generally made up of barium nitrate and antimony sulfide; however, most .22 ammunition does not have antimony and rare brands have neither antimony or barium. It also contains lead styphnate, but unfortunately for all of us, lead is so commonly found in the environment that the presence of it on someone’s hands is not considered significant. Therefore most GSR detection techniques concentrate on barium and antimony.

In all ammunition the ratio of barium to antimony is not known and manufacturers do not see fit to release that information. It is, understandably enough, their livelihood, and they are not interested in giving their formulas out. In addition, as in any other manufacturing process, they are often changing, adjusting, and experimenting with their batches.

Most gunpowder used today is ‘double-base’, which means it is a combination of nitroglycerin and cellulose nitrate (commonly and incorrectly known as nitrocellulose).