What causes bullets to keyhole?

The two usual cuplrits are barrel diameter and twist rate. If the barrel is larger in diameter than the bullet it can keyhold as it isn't being properly stabalized. The twist rate if if the twist rate is to slow and the bullet weight to heavy than what can happen is it will start to tumble due to not being spun fast enough to remain stable. There is also the opposite. If you use to light a round in to fast a twist the bullet will literally spin itself apart in midair.