Mil-spec buffer retainer detent and spring set for the AR-15 lower receiver — a small but essential pair of parts that keeps the buffer and recoil-spring assembly captured at the rear of the lower. The detent (a small plunger) and its spring sit in the buffer-retainer hole at the back of the magwell tower, pushing up on the front of the buffer so it does not pop forward when the upper is hinged open during cleaning or charging.
Common replacement small part for new builds and for repairing rifles where the original detent went missing during a buffer-tube install. The spring is correct length for mil-spec geometry; using a non-spec spring will either fail to retain the buffer or bind it in place.
Install with the spring in the receiver hole first, detent on top, then thread the receiver extension (buffer tube) far enough forward to capture the detent before backing off to align the castle nut and end plate. Press down on the detent with a punch as you thread the tube — do not let the detent flip out and fly across the bench.
