so ive been watching tv shows were bullets could be traced back to the exact gun used in shooting or crime. How does that work wouldn’t they need the gun that was used tho to track. or could they proceed with just a bullet
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When a gun is fired, it stamps a signature onto the bullet casing. That signature is used to help identify the gun.
[https://en.wikipedia.org/wiki/Microstamping](https://en.wikipedia.org/wiki/Microstamping)
Incidentally, it’s part of why there is heightened concern about ‘ghost guns’ (https://en.wikipedia.org/wiki/Privately_made_firearm).
They can’t look at a bullet and see a serial number or something on it of the gun that fired it, but what they can do is compare the markings on a bullet found at a crime scene and one fired from a suspected murder weapon to see if there’s a match. But they need to have the suspected gun present to make that comparison. So the police might have a suspect, who owns a gun of the same type that the murder was committed with (same kind of bullets) so they get a warrant to test that suspect’s gun to see if it matches the crime scene
Or they can look at a database of bullets that have been analyzed from prior crimes and say with some certainty that this new bullet was fired by the same gun used in X crime all those years ago, or whatever.
The answer from u/sudoku7 on microstamping is the right one. Think of it like the gun stamping on the bullet: “I am the weapon that fired this bullet”.
What you might see in shows like CSI (I am starting to feel old here) where they match rifling patterns on bullets to a specific weapon, but that is far from being a foolproof method, it has its limits. In some cases, it can identify from which weapon a bullet was fired and in others, not so much.
They can determine caliber fairly easily, they have the bullet that they recovered so they can just measure it. They take all the guns they think might be it and shoot them. They compare the marking on the bullets they fired to the one they recovered. Unless they already have your guns ballistics ( a sample stored at a local sheriff for ex) they must have your gun to do the tests to trace it to you. That’s why criminals ditch them in rivers.
Identifying bullets isn’t like DNA evidence even though CSI type shows makes it out to be. And prosecutors and police departments continue to reinforce.
It’s more that gun was used at crime scene. Gun left behind fired casing. And bullet in person. Some dude gets picked up down the street. His gun is loaded with the same ammo from the same lot. With the same bullet type.
People have been saying “marking on the bullet” or “rifling” and I’d thought I’d just explain what they mean by that. These markings on the bullet, which is called rifling, are scratches left on the bullet when it is shot out of the barrel.
When the police get a gun they think was the same one, they shoot a bullet out of it and compare the scratches on it left by the barrel to the bullet at the crime scene. If it shows the same scratch marks that proves it was the same gun.
First off, they can’t. The best they can do is say “a gun like this fired the bullet”
There’s a couple of steps involved. Guns fire different kinds of bullets. Some are long, some are short. Some are fat some are skinny. Using the combo of length, width, weight, and a couple of other things they can narrow it down to a few kinds of guns. Its kind of like using a tire to narrow down to a type of car. Different sizes of tires fit on different cars.
Each type of gun has a different style of barrel, most have something called rifling. That’s a set of grooves that spin the bullet so it goes straighter. There’s a different number of grooves, different depth, and different spin for different guns. That can narrow it down even further, either to one gun or a set of guns that use the same barrel.
But like tires this only narrows it down to a range of guns. With modern manufacturing there aren’t enough differences to reliably know which gun fired the bullet. Just like knowing that a car with a Michelin XGV size 75R14 tire narrows it down to a GM car you can’t tell if it was a 64 Skylark. You need additional information that’s not part of the tire to know it was actually a 63 Tempest.
It can depend on what evidence is recovered. If it’s the slug and it’s in decent enough condition (not completely fragmented) they can examine the pattern of the rifling and can determine caliber by examining the slug. Not all rifling is the same – some guns use polygonal rifling (like Glocks) that can narrow down the list of possible guns. The number of grooves, the spacing, and in which direction the twist of the rifling can help narrow it down too.
For the cartridge, there’s the marks on rim from the extractor, as well as the scuffing pattern across the head stamp from the breach face, as well as the marks left from ejection. They can tell if a 7.62×39 round was fired from an SKS, for example, due to distinctive marks. You also get an impression from the firing pin, which can narrow down the possibles. You can determine who originally made the ammunition as well, if it was factory made, and then can track down possible retailers.
Then if a gun is recovered, test shots or castings can be made and compared to what evidence you have. Comparison microscopes allow both the test slug and evidence slug to be compared, and there are data bases where breachface, firing pin strikes, etc are saved to be compared to recovered guns in the hopes of making a match and closing a case.
Just keep in mind most forensic science is junk science. Polygraphs, hair analysis, bite mark analysis, blood spatter analysis, handwriting analysis, hair analysis are all pretty shitty. Hell, even DNA evidence isn’t 100% reliable.
The problem is that:
1) law enforcement has a vested interest in blaming the crime on someone, even if they didn’t get the right guy.
2) many judges and law enforcement folks don’t have enough technical know how to be able to understand the nuances of what a test can and cannot “prove.”
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