CMMG MK4, Semi-automatic, Stripped Lower Receiver, 223 Remington/556NATO, Cerakote Finish, Armor Black 55CA102-AB Precision machined from forged 7075 T6 aluminum, the CMMG Mk4 AR-15 stripped lower receiver is the quality mil-spec lower ready to be the foundation for your next build. The Mk4 stripped lower comes standard in a Armor Black finish and can be assembled using any mil-spec Lower Parts Kit.