This upper is compatible with Mk4/standard AR15 lower receivers (when used with CMMG 9AR conversion mags), as well as MkGs and Mk17 lower groups. Its no secret that pistol stabilizing braces have been a target for scrutiny over the years, and those that enjoy building their own AR pistols have wondered what will come out of all the uncertainty. DISSENT is CMMG's answer. Utilizing CMMG's patent pending Compact Action and Radial Delayed Blowback technology with fixed ejector, 9mm DISSENT upper group houses the entire action and recoil assembly in the upper receiver, resulting in less bulk and length than traditional AR pistols. The forward non-reciprocating side charger allows for quick bolt operation without coming off target. DISSENT Upper groups include a 1913 Picatinny rail end plate to use in place of the lower receiver buffer tube.