Rebated Boat Tail
Also: RBT
A boat tail design with a stepped-down base behind the bearing surface, intended to ease engraving into the rifling and tune base drag.
A rebated boat tail starts with a conventional boat-tail taper but adds a small step where the shank meets the tail, so the rear of the bullet is slightly smaller in diameter than the bearing surface ahead of it. That rebate at the bullet-base is meant to give the rifling a cleaner start as the bullet engraves and to control how the airflow leaves the base. Bullet makers have used the geometry to chase both easier loading and a particular drag signature.
The aerodynamic effect is debated, and a rebated tail does not automatically raise ballistic coefficient compared with a plain boat tail of the same length. Its value tends to show in manufacturing consistency and in how the base interacts with muzzle gases at exit, rather than in a dramatic drag reduction downrange. As with any base form, the payoff is judged by group size and velocity consistency on paper, not by the profile alone.