CMMG M4/AR15 Assembled Upper Receiver - Expertly crafted from aircraft grade aluminum, CMMG M4/AR15 upper receiver assemblies come with our ZEROED Polymer Dust Cover pre-installed. This upper receiver omits the forward assist for those looking for a sleek and stylish base to start a new AR15 build. PCC Compatibility Featuring an enlarged ejection port, the CMMG Mk4 upper receiver can accommodate a variety of pistol calibers like 9mm, 10mm, .40 S&W, and .45 ACP and is compatible with CMMG MkG, MkGs, Mk10, Mk9, and Mk17 lower receivers. NOTE This upper is not compatible with CMMG fixed ejector Radial Delayed Blowback BCGs. Features Manufacturer: CMMG; Model: Mk4/AR15; Product Type: Upper Receiver Assembly; Material: Aircraft-grade Aluminum; Unique Features: ZEROED Polymer Dust Cover, Omitted Forward Assist; What's In The Box: Upper Receiver Assembly;