Keyholing
Also: Tumbling
Oval or sideways holes in paper made by an unstabilized bullet, almost always a sign of a twist rate or stability problem.
Keyholing describes the elongated, oval, or fully sideways holes a bullet punches in paper when it arrives without spinning fast enough to fly nose-first. Instead of a clean round hole, the projectile is tumbling or yawing badly, which wrecks accuracy and bleeds velocity through extra drag. The usual cause is a twist rate too slow for the bullet’s length, leaving the stability factor below the threshold needed to hold the nose forward.
Diagnosing it starts with the bullet and barrel pairing rather than the load, since a longer or heavier bullet may simply need a faster twist than the barrel provides. A load that is only marginally stable can keyhole at distance even when it prints round up close, because the bullet loses spin slower than it loses speed. The opposite failure mode, overstabilization, does not keyhole but carries its own small penalties.