Wednesday, July 11, 2012

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?

8 comments:

  1. I love the bullet match and primer impression match at trial. Even better than DNA. The jury gets to see the match for themselves in pictures, unlike DNA.

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  2. Exactly. You can actually see the jurors lean forward and tune right in to this stuff. And Greg, our main firearms guy, is such a great teacher, you can tell he loves doing it and so is very good.

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  3. I like the fact that such photos allow the lay person to formulate their own opinion of the evidence.

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  4. Anon, that's precisely why I like this evidence. The firearms folks explain what there is to look for and the jurors can see it (or not) for themselves. As PD said, in that regard it's even better than DNA.

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  5. Is it heard of - and if so, how common is it - that two guns (same maker, same model) could leave marks on the casings that look identical when analyzed microscopically?

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  6. I've done quite a few mock trials where Travis County in the State of "Lone Star" is our hypothetical jurisdiction. Hopefully I can finally start putting that (and this awesome info) into practice in a few months. For now, back to studying for the bar.

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  7. Okay. I want to be sure I have this correct. If two rimfire cartridges are fired from the same gun (at different times and locations), both empty casings will have the same marks on the rim--under microscopic observation. Here's why I want to know. I'm at the end of a 4-year journey writing a novel. With no murder weapon, if one empty casing is found at the crime scene, and another empty casing is found in the perpetrator's home, and they both have the exact same markings, it could be concluded that the perpetrator shot both cartridges from the same gun. This could prove a case without having the murder weapon. Please let me know if this is an accurate assumption. THANK YOU!!

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    Replies
    1. Ann, I hope my reply isn't too late. The answer to your question is... Yes and No. The Yes part: an expert could conclude that the rounds were both fired from the same gun, yes. The No part is your extrapolation - nothing about an empty casing will tell you who fired it. You probably know that, just be careful - a defense lawyer will argue that the casing was dropped there by the "real" perpetrator.

      But I think your main concern is met: if the have the same markings, they will have been fired from the same gun. Which is pretty good evidence in the absence of the weapon itself. :)

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