Our BCM MK2 Receiver Extension for your AR15, M16, M4. 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)(TM), Colt, LMT, SOPMOD, VLTOR, Stag Arms, and others that a made to the Mil-Spec size. THESE RECEIVER EXTENSIONS CAN ALSO BE USED ON MOST AR-10s.*NOTE* These buffer tubes require a M16 rifle action spring and specific BCM MK2 buffer, carbine and rifle buffers will NOT work.