For your AR15, M16, M4. Carbine receiver extension (buffer tube) machined from 7075T6 and manufactured to Mil-Spec dimensions and specifications. Mil-Spec receiver extensions have a slightly smaller dimension where the stock slides on. Hard Coat Anodized per Mil-A-8625F, Type III, Class 2 These tubes should be used with stocks such as USGI, BCM(R), Colt, LMT, SOPMOD, VLTOR, Stag Arms, Magpul CTR (Milspec), and others that a made to the Mil-Spec size.