CARBINEMILSPEC RCVREXT BUFTUBE 6POSITIONMil-Spec Carbine Receiver Extension Black - Fit: AR-15, M16, M4 with stocks such as USGI, BCM, Colt, LMT, SOPMOD, VLTOR, Stag Arms, Magpul CTR (Mil-spec), and others that a made to the Mil-Spec size - Hard Coat Anodizedothers that a made to the Mil-Spec size