LBE Unlimited ARCNUT Castle Nut 4140 Steel Black Phosphate AR-15

The LBE Unlimited ARCNUT Castle Nut is a precision-machined AR-15 lower receiver part built from rugged 4140 steel and finished in a durable black manganese phosphate coating. This is the castle nut responsible for locking the buffer tube, buffer, and action spring assembly securely to the rear of your AR-15 lower receiver, making it one of the most critical small parts in any AR build or maintenance job.

LBE Unlimited machines the ARCNUT from high-strength 4140 steel so it can stand up to repeated staking and the torque required to keep the buffer tube properly indexed. The manganese phosphate finish matches the factory black phosphate appearance found on mil-spec buffer tubes and receivers, giving your build a clean, consistent look while adding corrosion resistance for long-term service.

Designed as a direct-fit replacement for a worn or damaged castle nut on any standard AR-15 or mil-spec pattern carbine/rifle lower, the ARCNUT accepts a standard castle nut wrench and is compatible with the LBE Unlimited Mil-Spec 6-Position Buffer Tube and other mil-spec buffer tube assemblies. Whether you are completing a fresh build, replacing a stripped nut, or simply carrying a spare for the range bag, this LBE Unlimited castle nut delivers the made in USA reliability and quality you need.