The CMMG Resolute Upper Group in 5.7x28mm (brand part number 57B40E4AB) is a complete 16.1-inch upper assembly for the Mk4/Mk57 platform, built to deliver PDW-class ballistics with the familiar handling of a full-size carbine. It runs CMMG’s Radial Delayed Blowback operating system for reliable cycling with a fixed ejector, and it comes complete with a bolt carrier group and CMMG’s ZEROED accessories: a polymer dust cover, ambidextrous charging handle, and 5.56mm-pattern muzzle brake. Pair it with a CMMG Mk57 lower or a standard AR-15 lower along with CMMG’s 5.7 AR conversion magazines to build a fast-handling personal-defense carbine.

Key Specifications

• Chambered in 5.7x28mm, a personal-defense caliber known for high velocity and low recoil.
• Barrel length measures 16.1 inches, clearing the federal rifle minimum with no NFA paperwork required.
• Rate of twist is 1:9, matched to 5.7x28mm bullet weights for reliable stabilization.
• Operating system is CMMG’s Radial Delayed Blowback with a fixed ejector, tuned specifically for this caliber conversion.
• Barrel contour is MT, machined from 4140CM steel for durability.
• Barrel finish is salt bath nitride (SBN), a corrosion-resistant coating.
• Muzzle device is the CMMG ZEROED Muzzle Brake, threaded 1/2×28 for reduced muzzle rise.
• Upper receiver is billet 6061-T6 aluminum with a CMMG EML15 M-LOK hand guard for accessory mounting.
• Ships complete with a bolt carrier group and CMMG ZEROED ambidextrous charging handle, ready to install on a compatible lower.

Frequently Asked Questions

Is this a complete firearm? No, this is an upper receiver group only. It ships without an FFL transfer and pairs with a CMMG Mk57 lower or a standard AR-15 lower to complete a rifle.

What magazines does this use? It runs CMMG’s 5.7 AR Conversion Magazines, not standard AR-15 magazines, due to the 5.7x28mm cartridge geometry.

Does this include the bolt carrier group? Yes, all CMMG Upper Groups ship complete with a bolt carrier group and CMMG’s ZEROED accessories, so no separate BCG purchase is needed.

Buy the CMMG Resolute 5.7x28mm Upper Group

This upper group will ship as soon as it’s available from a Lubbock, Texas gun shop for your next PDW-caliber build. Browse more complete uppers in our AR15 uppers selection.