An injury to the muzzle of a rifle can cause shots to keyhole. This condition is most likely to occur in rifles cleaned from the muzzle, but may be due to accident, such as striking the muzzle against any metal.
A very small burr on the muzzle crown can make the bullets unstable in flight.
Anything that reduces the rotations per second may cause a bullet to tip. Low bullet velocity may reduce the spin enough to cause this effect. A long bullet requires more spin than a short one, so that too long a bullet for the twist or too slow a twist for the bullet may cause tipping. Excessive wear of the rifling or too loose a fit of the bullet in the barrel may cause the bullet to fail to take the full spin required.
As barrel erosion progresses, the portion of the barrel just …