Mil-spec steel castle nut for AR-15 lower receivers — the threaded retainer that locks the receiver extension (buffer tube) to the back of the lower and traps the end plate against the castle face so the rear takedown detent spring stays seated. The castellated notches on the rear face accept a standard armorer's wrench for torquing, and the same notches let you stake the nut to the end plate after install so it won't back off under sustained fire.
Confirm your receiver extension is mil-spec — commercial-diameter tubes use a coarser thread and a slightly different castle nut. Torque to 38–42 ft-lbs with a proper castle nut wrench, then stake at two opposing notches into the end plate so it cannot rotate loose. Never substitute a non-staked castle nut on a duty or hard-use rifle.
