eli5: How can the police find the gun just by having the bullet that’s been shot from it?
In: Engineering
Well first thing they can do is tell what type of gun fired the bullet by the bullet size. (i.e a 9mm bullet was fired from a gun that fires 9mm bullets so they know what type of guns their looking for). Next when they find a gun they fire a bullet from this gun. In the barrel of every gun is corkscrew like grooves which make the bullet spin as it leaves the chamber. This leaves an imprint on the bullet which is individual to that particular gun, think of it like a thumbprint. So they test fire the gun they think was used in the crime and then compare it to any bullets found at the scene. If the bullets match then that’s your weapon and the case goes on from there
They can’t. Just having the bullet tells you almost nothing useful. However, they can use that with other information to locate a gun. Perhaps that gun has been used to commit a crime before, and it was sold at auction; there will be a record of that. So the ballistics from the previous crime and the auction data could provide a starting point. Or perhaps you don’t just have the bullet, you have it embedded in a wall; tracing that damage back through other points, like a broken window, could lead to where the shot was fired from; perhaps the shooter abandoned the gun there or close by. Or perhaps the bullet itself is unusual; for example, it may be a .40S&W with a jacket swaged from 9MM brass. That’s almost certainly the product of a handloader, so they might start asking around about people who do custom handloading, or who have purchased bullet swaging equipment. That could lead them to who made the round, which could lead them to who bought and fired it.
They can’t “find” the gun, what they can do is test a gun and see if it makes the same marks on a new bullet as does the old bullet the marks or lands and grooves are created as the bullet moves down the barrel of the gun and are unique to each gun.
You can’t find the gun, but if a suspect gun comes into your possession, you can match it up. Like fingerprints.
Seemingly identical guns are nearly but not perfectly identical, because they exist in the real world and were created by real machine tools.
The microscopic manufacturing variations leave their mark on the round during it’s brief, tortuous birthing process through the barrel of the weapon.
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