C.S.I. cartridge case science

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moooose102

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ok, i openly admit i watch to much t.v. . one of the shows i watch regularly is CSI. when they are talking about matching a casing to a particular gun, how is that done? i can understand how the extractor marking, and the fireing pin markings are there. but how can they be certain that it belongs to that exact gun? i can see relation between the two. say the firing pin is a little off center, and the extractor is at the 3 o clock position. but wouldnt there be many of that type of gun that would leave similar markings? and what if a person reloads, the case in question would have multiple extractor markings on it. how would they be able to tell with any certainty that the case belonged to that gun, especially if it were range brass? it just seems far fetched to me that every gun leaves completely individule markings on a case that are so different that they would be as good as fingerprints.
 
The firing pin, ejector and extractor can be matched just like rifling. You never hear about it in real life because if they pull a slug out of a victim, and its a match to the suspect's gun, there's really no need to match the brass as well.

The lady is a CSI junkie, so I get stuck watching it, as well as the re-runs on spike. One night someone picked up some brass and I said "Looks like a .50 action express, dude got shot with a desert eagle." So she started to roll her eyes and it showed the headstamp. .50 AE. Just thought that was cool...
 
...I said "Looks like a .50 action express, dude got shot with a desert eagle."
Click to expand...

Unless, of course, it was shot out of one of the other guns that fire that cartridge.
 
in the real world, they would magnify the 'toolmark' left by the firing pin to a degree where what appears smooth and even is shown to be otherwise. They then take another case fired from the same gun, and examine it, if it is the same gun, in theory, there should be a whole bunch of marks that are identical within the primer strike.

Note, you cannot identify the general type of gun from such a mark. Once a specific gun is found (or just a cartridge) that gun can be matched to the cartridge in evidence.

of course, shooting a few thousand rounds between the evidence case and the gun being taken for the test would wear the firing pin enough that it wouldn't show up as the same. Or just take a file to the firing pin.

This is the same technology used in matching bullets based on marks left by the rifling.
 
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