The CMMG RESOLUTE 5.7x28mm Upper Group, 16.1-Inch (CMMG part no. 57B40E4AB) brings a personal defense caliber to a carbine-style upper. CMMG pairs it with a Radial Delayed Blowback operating system, which it says gives a softer recoil impulse and reduced bolt weight. The 16.1-inch barrel has a 1:9 twist and a ZEROED RBB Brake threaded 1/2×28. It runs on either a CMMG Mk57 lower or a standard AR15 lower group and comes in Armor Black Cerakote.

Key Specifications

• The chambering is 5.7x28mm, a PDW cartridge.
• The platform is Mk4 and Mk57, so it fits an Mk57 lower or a standard AR15 lower group.
• The barrel length is 16.1 inches with a 1:9 twist and a medium taper contour.
• The barrel is 4140 chrome moly with CMMG’s SBN finish.
• The muzzle device is a ZEROED RBB Brake, threaded 1/2×28.
• The operating system is Radial Delayed Blowback, a design CMMG says reduces bolt weight and recoil.
• The upper receiver is billet 6061 T6 aluminum, a light and rigid alloy.
• The handguard is a CMMG EML15 M-LOK, which accepts M-LOK accessories.
• The charging handle is a CMMG ZEROED ambi model that works from either side.
• The upper group weighs 4.0 pounds and measures 25.3 inches long.

Frequently Asked Questions

What lowers does it fit? CMMG says to combine this RESOLUTE upper group with either its Mk57 lower or a standard AR15 lower group.

What is Radial Delayed Blowback? It is CMMG’s operating system, adapted from its .45 ACP MkG platform. The company says it gives a softer recoil impulse and reduced bolt weight compared to traditional straight blowback.

Does it come with a bolt and charging handle? Yes. CMMG says its upper groups come complete with a bolt carrier group and a ZEROED ambi charging handle.

Buy the CMMG RESOLUTE 5.7x28mm Upper Group, 16.1-Inch, ZEROED RBB Brake, Armor Black

This is a ready-to-run 5.7x28mm upper for the lower you already have. Browse more AR-15 uppers from CMMG. Order from our Lubbock, Texas gun shop for fast, direct shipping.