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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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 You can determine who originally made the ammunition as well, if it was factory made, and then can track down possible retailers.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.
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