CMMG Mk4 Resolute Upper Receiver Group 6.5mm Grendel 16.1in Armor Black. Chambered this upper group is a staple for any collection and comes ready to pair with the AR15 lower group of your choice. Specifications for CMMG Mk4 Resolute Upper Receiver Group: Manufacturer: CMMG Muzzle Device: CMMG Zeroed Muzzle Brake Color: Armor Black Fabric/Material: Billet 6061 T6 Aluminum Gas Block Journal Size-Diameter: .750 in Gun Rail: M-LOK Gun Type: Rifle Gun Make: CMMG Gun Model: CMMG Mk4 Condition: New Magazine Included: No Package Contents: CMMG Mk4 Resolute Upper Receiver Group What firearm models does the manufacturer say CMMG Mk4 Resolute Upper Receiver Group is compatible with? The manufacturer suggests that CMMG Mk4 Resolute Upper Receiver Group is compatible with CMMG Mk4CMMG 64BCF88-AB: Mk4 Resolute Upper Receiver Group 6.5mm Grendel 16.1in Armor Black