The CMMG 16.1-Inch Barrel and BCG Kit (brand part number 57D0476) is built for a carbine-length AR chambered in 5.7x28mm, using CMMG’s patented Radial Delayed Blowback operating system. Unlike a typical direct blowback design, Radial Delayed Blowback reduces the weight of the bolt carrier group and produces less felt recoil than a straight blowback system, all without sacrificing key features like last-round bolt hold-open. This threaded, black-finished 16.1-inch barrel and bolt kit is designed to be assembled into a Mk4/mil-spec AR15 upper and works with either a CMMG Mk57 lower receiver or a Mk4/AR15 mil-spec lower receiver when used with CMMG’s 5.7 AR conversion magazines.

Key Specifications

• Chambered in 5.7x28mm, a fast, flat-shooting pistol-caliber cartridge.
• Barrel length is 16.1 inches, sized for a carbine-length AR build.
• Muzzle is threaded, ready for a muzzle device.
• Operating system is CMMG’s patented Radial Delayed Blowback, reducing BCG weight and felt recoil versus straight blowback.
• Retains last-round bolt hold-open functionality.
• Finish is black.
• Compatible with a CMMG Mk57 lower or a Mk4/AR15 mil-spec lower when used with CMMG 5.7 AR conversion magazines.
• Sold as a matched barrel and bolt carrier group kit.

Frequently Asked Questions

What lowers does this kit work with? It works with a CMMG Mk57 lower receiver or a Mk4/AR15 mil-spec lower receiver when used with CMMG 5.7 AR conversion magazines.

Does it retain last-round bolt hold-open? Yes, unlike some blowback systems, this kit retains last-round bolt hold-open.

What does Radial Delayed Blowback do? It reduces the weight of the bolt carrier group and produces less felt recoil than a typical straight blowback system.

Buy the CMMG 5.7x28mm Carbine Barrel Kit

Build a carbine-length 5.7x28mm AR with CMMG’s proven blowback system. Shop more CMMG parts in our AR15 barrels selection, and this kit ships fast from our Lubbock, Texas gun shop.