You hear all the time about police matching a bullet to a gun. I can understand them being able to determine the caliber of a bullet, but there are perhaps millions of guns in more common calibers in the states. How do they match a bullet with an exact gun?
Forensic ballistics can only match a particular individual gun to a bullet if they have
- The bullet from the crime scene
- The gun suspected of shooting that particular bullet
What they do is they shoot a new bullet from the gun and compare the marks on this bullet it to the marks found on the bullet from the crime scene.
How does this work with shotgun slugs?
My understanding is that shotgun firing pin locations vary, as do the scrapes on the shell casing. I think is more the shell than the round in that case.
Okay, so what if the bullet distorts from hitting a bone? Or what if it’s a hollow point and spreads out upon impact?
Shotgun slugs would be able to have similar balistic testing done as standard rifle/pistol rounds.
The case matching would be used in shotgun cases where some sort of “shot” was used… anything from 000 buckshot (biggest) through any of the bird/small game shots using smaller pellets.
The pin strikes can be very distinctive, having location, shape, and depth components… in addition to any small variations in the acutal striking surface of the pin itself.
I’ve been informed that matching slugs fired from one of the pistols that use polygonal rifleing, Glocks and several others, is virtually impossible. These types of rifleing do not create the distinct grooves that the more traditional land and groove system does.
If a person has access to extra barrels, that would also defeat the system.
There are schemes that collect a fired case from all new pistols sold in a state. These are entered into a database. The idea is that a case found at a crime scene could be traced back to a particular pistol. A fired case carries marks from the fireing pin, as well as imprints from the face of the bolt. In practice, this has not proved to be a workable system, in spite of the vast amounts of money thrown into the system.
Shotguns are smooth bore–no grooves. So, how could there be a ballistics test? 
Valid. My brain cramp was thinking rifled barrel. Can you tell I’m shopping for one for my Mossberg 500?
You can have either a smooth bore, which you use standard foster (I think) slugs with. Or a rifled barrel, which you could use foster slugs with, or a sabot round. In the sabot round’s case, I’d assume the balistic evidence would be on the sabot, not on the actual slug.
It’s early, and not enough caffeine has hit my system! :smack:
If the bullet is ruined to a large enough extent, then you’re right, it can’t be matched very well. But, at least for hollow points, it is sometimes possible, I would imagine, to “fold back” the points, as it were, and get the rifling marks. Plus, you don’t need to whole bulletm, just enough of it has to be intact to get the marks, so if it hit a bone, odds are only the first 50-75% of it will be too distorted to use. But still, there is the occaisional bullet that cannot be matched using forensics.
It’s possible to match bullets or slugs fired from rifled barrels, and it’s possible to match ejected cases.
Rifling leaves microscopic scratches in jacket metal, and two slugs fired from the same gun will show identical patterns (and hollow points do often break up to the point where matching is impossible). There’s nothing particularly special about polygonal rifling. Lead is tougher to match than jacket metal.
Ejected casings will have similar patterns left by the extractor. You can’t match shotgun slugs, but you can match fired hulls.
Keep in mind, however, that matching is only possible if a very small number of rounds are fired between the samples you want to match. Firing, say, a hundred rounds between samples will wear the parts enough to change the patterns. That’s why “ballistic fingerprinting” is worthless.
100 rounds or 30 seconds with a swiss file!