Boat Tail
A bullet base that tapers inward toward the rear rather than cutting off flat. The taper reduces base drag and raises ballistic coefficient.
Behind a supersonic bullet, the air can’t fill the void left by a flat base fast enough. Pressure drops there and drag goes up. A boat-tail’s slope lets the airflow stay attached longer, easing the wake.
For inside 300 yards, the difference is negligible. Past 600, boat-tails win on retained velocity and on wind. Past 1,000, flat-base bullets become a specialized choice that you mostly see in certain hunting roles.