Ballistics - matching casings

One of the coolest aspects of this job is getting to work with people who are masters in their field. Of course, given my job their field is inherently interesting so you can imagine how fun it is to work with, say, a top-notch firearms analyst.

You know, like Austin P.D.'s Greg Karim.

Two of the things Greg and his colleagues can do include tests to (1) determine whether or not a particular shell casing was fired in a particular gun; and (2) whether or not a particular bullet was fired from a particular gun.

I want to do two posts on this, one for each. You'll see why, below. First the casings, and we should be sure of terminology. Here are the terms we use:

Cartridge -- what you pull from the box of ammo you buy at Wal-Mart, and load into your gun.
Bullet -- the piece of lead that is expelled from the barrel of the gun when you shoot.
Casing -- the container for the propellant and bullet, which is left empty after use and either expelled from the gun (via ejector) or remains in the cylinder.

Here's how he works: a spent casing is found near the body of our imaginary victim. It's placed into evidence and locked away. A gun is found on a suspect, who's now a hundred miles from the crime scene. It is also placed into evidence and locked away.

The essential question is: was the casing found near the victim fired in this gun? If so, that's some good evidence. If not, we need to know that too, so we can find the right murder weapon.

So, Greg retrieves the casing and the gun from the evidence locker and retreats to his lab. There, he fires the weapon into a barrel of water, using identical ammunition to the found casing. He'll do a number of test fires, fifteen for example. Then he will put the casing from the crime scene under a microscope, examine it, and take close-up photos. He will do the same with the test-fired casing. I am simplifying, and he'd pull his hair out at such a poor summary of his laborious, time-intensive work, but this is essentially the process by which he can say whether or not the casing from near the body was fired from the gun found on the suspect.

What he's looking for are areas on the two casings that bear the same marks, showing that when you fire this type of ammo from this gun, you'll get this mark. The marks can come from the casing expanding inside the barrel (when it gets hot) and essentially being 'printed' with the marks in the barrel, and the marks can come from the firing mechanics themselves.

You know what? Best if I show you . . .

Pretty neat, huh? As you've probably figured out, these photos show the hypothetical casing from the crime scene (left) and the test-fire (right). They sure look like a match, but can we do better? Can we be more certain? Why yes, since you ask, we can. . .


The tiny black line down the center is where the subject casing and the test-fired casing meet. This is the photo that always impresses jurors (and me!) because you can see for yourself how the two match up.

Here are some more from other casing analyses (credit where it's due, these photos were actually taken by Erica Pusch, Greg's colleague at APD).


And coming up soon, images like this of the bullets themselves. Pretty neat, don't you think?