AR-15 BUFFER TUBE CASTLE NUT. A castle nut is a type of nut with slots, or notches, cut into one end that resembles the battlements of a castle. It is used in low-torque applications. This prevents the nut from loosening due to vibration or movement, and common use include securing components on firearms. Common applications: Firearm components, such as the buffer tube on an AR-15 and other low-torque assemblies that experience vibration. How it works: Tightening: The castle nut is tightened onto a bolt or buffer Tube. Safety: It is crucial for safety in applications where a nut failure could cause a hazard. Reusability: The nut itself can be reused.