MIL-SPEC AR15 castle nut is built from high quality steel and Manganese Phosphate coated. A Mil-Spec AR-15 castle nut might be an after-thought to most AR builders but it is an essential component for keeping your rifle running reliably. The castle nut simply slides over the receiver extension/buffer tube and is screwed on to secure the entire buffer and stock system to the lower receiver. This connection between the buffer system and receiver is generally regarded as one of the weak spots on the AR platform and this Mil-Spec castle nut will ensure your buffer tube is solidly attached for thousands of rounds. Especially if you stake the end plate resulting in a mechanical connection between the two pieces of metal and preventing the castle nut from loosening over time.