Bullet Base
The rear face of a bullet. Its shape, flat or tapered, is the single biggest driver of base drag and downrange ballistic coefficient.
The base is where the air closes back in behind a bullet in flight, and that wake is a large share of total drag. A flat base cuts off square, leaving a low-pressure void the airflow cannot fill cleanly at supersonic speed. A boat-tail tapers the base inward so the flow stays attached longer and the wake shrinks.
The base also matters at the muzzle. A square, undamaged base leaving an even crown tips off uniformly, while a nicked or canted base gets an asymmetric gas push that starts the bullet yawing. This is why precision shooters guard the crown and inspect bullet bases.