system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: How can the police find the gun just by having the bullet that's been shot from it?
gpt
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.