CMMG Upper Receiver Assembly Mk4/AR15 Forward Assist Sniper Grey. Available in six different Cerakote finishes this mil-spec workhorse has been proven over the years and makes a great base to build your next AR15 upper group. Specifications for CMMG Mk4/AR15 Upper Receiver Assembly w/Forward Assist: Manufacturer: CMMG Barrel Profile: M4 Fabric/Material: 7075 T6 Aluminum Gun Make: CMMG Gun Model: CMMG Mk4 Magazine Included: No Condition: New Package Contents: CMMG Mk4/AR15 Upper Receiver Assembly w/Forward Assist What firearm models does the manufacturer say CMMG Mk4/AR15 Upper Receiver Assembly w/Forward Assist is compatible with? The manufacturer suggests that CMMG Mk4/AR15 Upper Receiver Assembly w/Forward Assist is compatible with CMMG Mk4CMMG 55BA206-SG: Upper Receiver Assembly Mk4/AR15 Forward Assist Sniper Grey