Quite honestly poorly.
Studies in recent years have shown that ballistic analysis is highly flawed and unreliable.
But to answer your question. The barrel of a gun has grooves etched into it called rifling. These grooves are used to spin the bullet when it is fired. That spin makes the bullet travel straighter.
The theory is that if you have two bullets fired from the same gun, the grooves created by the rifling should be identical.
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