The CMMG 5.7x28mm Barrel and BCG Kit (brand part number 57DBC33) lets a builder assemble an AR-platform pistol in 5.7x28mm using CMMG’s patented Radial Delayed Blowback operating system. Unlike a typical direct blowback system, Radial Delayed Blowback reduces the weight of the bolt carrier group and produces noticeably less felt recoil than a straight blowback design. This 5-inch black barrel and BCG kit is designed to be assembled into a mil-spec AR15 upper and works with either a CMMG Mk57 lower receiver or a standard Mk4/AR-15 lower receiver when paired with CMMG’s 5.7 AR conversion magazines, giving builders flexibility in how they configure the finished pistol.

Key Specifications

• Chambered in 5.7x28mm, a fast, flat-shooting pistol-caliber cartridge.
• Barrel length is 5 inches, sized for a compact AR-platform pistol build.
• Operating system is CMMG’s patented Radial Delayed Blowback, which reduces BCG weight and felt recoil versus straight blowback.
• Finish is black.
• Compatible with a CMMG Mk57 lower or a standard Mk4/AR-15 lower when used with CMMG 5.7 AR conversion magazines.
• Sold as a matched barrel and bolt carrier group kit, not just a bare barrel.

Frequently Asked Questions

What lowers does this kit work with? It works with a CMMG Mk57 lower receiver or a standard Mk4/AR-15 lower receiver when used with CMMG 5.7 AR conversion magazines.

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.

Does a 5-inch barrel like this require an ATF Form 1? No, at 5 inches this is built for pistol configuration; installing a barrel this short on a rifle-configured (shouldered) lower under 16 inches would legally require a registered SBR, so keep this kit on a pistol-configured build.

Buy the CMMG 5.7x28mm Barrel and BCG Kit

Build a fast-cycling 5.7x28mm AR pistol with CMMG’s proven blowback system. Shop more CMMG parts in our AR bolt carrier groups selection, and this kit ships fast from our Lubbock, Texas gun shop.