CMMG Resolute MK4, Upper Receiver, 6MM ARC, 16.1 Barrel, Cerakote Finish, Armor Black 60B1091-AB Meet CMMG's newest long-range offering. Chambered in Hornady's new 6mm ARC (Advanced Rifle Cartridge)" the CMMG's RESOLUTE upper receiver group allows you to pair this exciting caliber with your existing AR15 lower for superior function and accuracy at extended ranges.