system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:  How does law enforcement match a bullet to a specific gun?

I get matching it to a model, but how do they verify the specific gun?  Do guns leave some type of ballistic finger print on rounds, and if so, is that fingerprint made on purpose or a natural result of minor manufacturing inconsistency?
gpt
All guns have a series of spiral grooves cut into the inside of the barrel. This groove pattern causes the bullet to spin which helps it fly straight.

This groove pattern is left on the bullet as it leaves the gun. Due to minor manufacturing inconsistencies the exact pattern is unique to every gun.

When running a ballistic test the police will test fire rounds from a given gun and compare the test bullets to bullets recovered at the crime scene. If the groove pattern matches up then there's a good chance it's the same gun.