CMMG AR-15 Resolute 300 Mk57 5.7x28mm Upper Receiver Assembly - 16" Barrel, M-LOK Handguard, Radial Delayed Blowback

The CMMG AR-15 Resolute 300 Mk57 5.7x28mm upper receiver assembly delivers modern PDW (personal defense weapon) ballistics in the handling and ergonomics of a classic AR-15 carbine. Built on CMMG's proprietary radial delayed blowback operating system, this 16-inch upper lets you convert a standard AR-15 lower receiver—or pair it with CMMG's dedicated Mk57 lower—into a nimble, controllable rifle chambered in the popular 5.7x28mm cartridge. Whether you're exploring a new caliber at the range or building a compact defensive platform, this upper arrives complete and ready to work out of the box.

The barrel is 4140CM steel with a 1:9 twist rate and finished in a tough MT contour. CMMG secures it with their ZEROED muzzle brake (1/2 x 28 thread) to manage recoil and muzzle rise. The upper receiver itself is billet 6061 T6 aluminum, a material choice that balances durability and weight. Mounted on top is CMMG's EML15 M-LOK handguard, offering modern rail real estate for lights, lasers, or furniture without excess weight or bulk.

Included hardware covers the essentials: the bolt carrier group is ready to run, a ZEROED ambi charging handle makes manipulations smooth from either shoulder, and a dust cover keeps carbon and debris out when the rifle is slung. The whole assembly weighs just 4.0 lbs and measures 25.3 inches end to end—compact and fast-handling. Available in standard Cerakote finish, it integrates seamlessly with most AR-15 platforms and accessories. Pair it with CMMG's 5.7x28mm conversion magazines and you're set to run a caliber that's become a serious option for shooters seeking lower recoil and hearing-safe ballistics without sacrificing terminal performance.