Cmmg Brl And Bcg Kit 4140Cm Sbn Drop In 5.7 X 28Mm 5" Black

The CMMG 5.7x28mm Barrel and BCG Kit gives you a drop-in way to build a 5.7x28mm AR without starting from scratch. This 5" barrel and bolt carrier group combo is built on CMMG's Radial Delayed Blowback operating system—a key difference from standard direct blowback designs. The result: a lighter BCG and noticeably softer recoil impulse, which matters when you're running a compact, high-velocity rifle cartridge designed for capacity and controllability.

The kit uses 4140 steel construction and comes finished in black. It's designed as a true drop-in upgrade for mil-spec AR15 upper receivers, so if you already have a quality upper sitting around, this barrel and carrier group pair handle the heavy lifting. The beauty of the 5.7x28mm platform is its low recoil and flat trajectory in a rifle-length setup, making it ideal for suppressed shooting, competitive rifle sports, or just a fun, accurate cartridge that's easy on the shoulder.

Compatibility is straightforward: the kit works with CMMG Mk57 lower receivers or with standard Mk4/AR-15 lower receivers when you pair it with CMMG 5.7 AR conversion magazines. Either way, you get a complete, functional 5.7x28mm rifle without the guesswork of sourcing individual components.

Key Features

• 5" barrel and bolt carrier group as a matched pair
• Radial Delayed Blowback system reduces felt recoil and BCG weight
• 4140 steel construction
• Drop-in compatible with mil-spec AR15 uppers
• Works with CMMG Mk57 lowers or standard AR-15 lowers (with CMMG 5.7 conversion magazines)
• Black finish

Specifications

Frequently Asked Questions

Will this 5.7x28mm barrel and BCG kit fit my existing AR-15 upper?

Yes, the kit is designed as a drop-in for mil-spec AR15 upper receivers. You'll also need a compatible lower receiver: either a CMMG Mk57 lower or a standard Mk4/AR-15 lower paired with CMMG 5.7 AR conversion magazines.

What's the advantage of the Radial Delayed Blowback system?

Compared to standard direct blowback systems, CMMG's Radial Delayed Blowback reduces the weight carried in the bolt carrier group and delivers noticeably less felt recoil—a real benefit in a cartridge designed for controllability and precision.

What magazines do I need with a standard AR-15 lower?

If you're using this kit on a standard Mk4/AR-15 lower receiver, you'll need CMMG 5.7 AR conversion magazines to feed the cartridge properly.

Is this a complete upper receiver or just a barrel and BCG?

This kit includes only the barrel and bolt carrier group. You'll need to install it into an existing mil-spec AR15 upper receiver or have one assembled separately.