Advanced Technology A5101020 AR-15 Castle Nut Mil-Spec Steel Black

The Advanced Technology A5101020 is a Mil-Spec AR-15 castle nut machined from steel and finished in a low-gloss black coating. Built to true Mil-Spec dimensions, this castle nut is virtually indestructible under the repeated torque demands of stock and buffer-tube service, making it an ideal drop-in replacement for a worn or stripped factory nut.

The A5101020 is designed to work with commercial, military, and hybrid buffer tubes, giving AR-15 builders flexibility across a wide range of lower receivers and buttstock combinations. It fits most AR-15s and AR variants, though it is not compatible with 308 rifles. The castle nut is the small but critical component that locks the buffer tube, buttstock, and endplate to the lower receiver, so upgrading it to a rugged Mil-Spec steel piece is a smart move for both build projects and field repairs.