CMMG 8in Barrel & BCG Kit — 5.7x28mm | Radial Delayed Blowback

Description

This CMMG kit pairs an 8-inch barrel with a matching bolt carrier group so you can build an AR in 5.7x28mm without hunting down mismatched parts. Everything is set up to drop into a Mk4 or mil-spec AR15 upper, which makes it a clean starting point for a compact pistol or short-barreled rifle project. The barrel is 4140 chrome moly with a 1:9 twist, a medium taper profile, and a salt bath nitride finish that holds up well against corrosion and wear. The muzzle comes threaded 1/2×28, so most 5.7-rated brakes, flash hiders, or a can will thread right on.

What Sets This 5.7x28mm Barrel and BCG Kit Apart

The heart of this kit is CMMG's patented Radial Delayed Blowback operating system. Instead of relying on a heavy bolt to slow things down like a traditional straight blowback gun, the radial delayed design lets you run a lighter bolt carrier while cutting felt recoil. That translates to a smoother shooting gun that stays flat and controllable, and it still keeps essential features like last round bolt hold open so you know when you're empty.

Running the CMMG 5.7 Kit on Your Lower

This kit is built to work two ways. Pair it with a CMMG Mk57 lower and you're good to go, or run it on a standard Mk4 or AR15 mil-spec lower when you feed it with CMMG's 5.7 AR Conversion Magazines. That second option is the big draw for a lot of builders, since it lets you keep your existing lower, trigger, and controls while adding a fun, high-velocity caliber to the mix. Keep in mind this setup calls for an enlarged ejection port on your upper for reliable brass clearance, and CMMG recommends a standard carbine buffer and action spring to keep the action cycling right.

The 5.7x28mm cartridge itself is a light, fast-moving round that was originally developed as a NATO answer to the 9mm. It brings a flat trajectory and minimal recoil, which makes a build like this genuinely easy to shoot at the range and light on the shoulder over a long session.