Bravo Company MK2 Receiver Extension, Fits AR, Buffer Tube, Matte Finish, Black BCM-MK2-RE 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. *NOTE* These buffer tubes require a rifle action spring and specific BCM MK2 buffer" carbine and rifle buffers will NOT work.