Seater Die
Also: Seating Die
The die that presses a bullet into a sized case to a set depth; competition versions use a sliding sleeve to keep the bullet aligned for low runout.
A seater die is the member of a reloading dies set that drives a bullet into the case mouth and stops it at an adjustable seating depth. A threaded stem with a contoured tip pushes on the bullet ogive, while the die body supports the case so the two stay in line.
Competition or micrometer seater dies add a spring-loaded sliding sleeve that captures the case and bullet together before seating begins, which holds them coaxial and reduces runout. Reloaders often check that result on a concentricity gauge, since a straight, well-aligned cartridge tends to shoot more uniformly.