The CMMG barrel and BCG kit (brand part no. 57D88C3) lets builders put together an AR platform chambered in 5.7x28mm, pairing an 8-inch barrel with a matching bolt carrier group built around CMMG’s patented Radial Delayed Blowback operating system. Unlike a typical direct blowback design, Radial Delayed Blowback reduces the weight of the BCG while delivering less felt recoil than a straight blowback system, making for a softer-shooting 5.7 build. The kit is designed to be assembled into a mil-spec AR-15 upper and runs with either a CMMG Mk57 lower receiver or a standard Mk4/AR-15 lower when paired with CMMG’s 5.7 AR conversion magazines.

Key Specifications

• Chambered in 5.7x28mm.
• Barrel measures 8 inches.
• Built around CMMG’s patented Radial Delayed Blowback operating system.
• Reduces BCG weight compared to a typical direct blowback design.
• Delivers less felt recoil than a standard direct blowback design.
• Designed to assemble into a mil-spec AR-15 upper.
• Compatible with a CMMG Mk57 lower.
• Also compatible with a standard Mk4/AR-15 lower using CMMG 5.7 conversion magazines.
• Finished in black.

Frequently Asked Questions

What lower receiver does this kit work with? A CMMG Mk57 lower, or a standard Mk4/AR-15 lower when using CMMG’s 5.7 AR conversion magazines.

Does this kit include the BCG? Yes, it includes a matching bolt carrier group built for the Radial Delayed Blowback system.

What makes Radial Delayed Blowback different? It reduces BCG weight and delivers less felt recoil than a typical straight blowback design.

Buy the CMMG Barrel and BCG Kit

Build a softer-shooting 5.7x28mm AR with a complete CMMG kit. Order it from our CMMG lineup, part of our AR bolt carrier groups selection at our Lubbock, Texas gun shop, and have it on hand as soon as it’s ready to ship.