Ergo 4971B Castle Nut AR-15/AR-10 Steel Black

The Ergo 4971B Castle Nut is a Mil-Spec CAR castle nut machined from steel and finished in black to secure the receiver extension (buffer tube) to the lower receiver on AR-platform rifles and pistols. It is engineered as a direct-fit, durable replacement for the factory castle nut, delivering the same reliable buffer-retention function expected of mil-spec hardware.

Built to fit AR-15 and AR-10 lowers as well as all AR variants, the 4971B works with Commercial, Military, and Hybrid buffer tubes, making it a versatile drop-in component for stock assemblies, retrofits, and spare-parts kits.

Installation is straightforward using a standard castle-nut wrench or armorer's tool. Whether you are assembling a complete lower, swapping a stock system, or replacing a worn or damaged nut, the Ergo 4971B provides consistent fit and a corrosion-resistant black finish in a compact steel package.