CMMG Receiver Extension Kit, For AR-15 Carbine, Black 55CA6C7 When you are thinking about building a new AR-15 rifle or completing that lower you have had sitting around this CMMG AR-15 Carbine Receiver Extension Kit 6-position Mil-Spec is the perfect one. This kit contains an AR-15 carbine receiver extension" end plate" carbine buffer and spring.