CMMG 16.1" 5.7x28mm Barrel and Bolt Carrier Group Kit

The CMMG 16.1" 5.7x28mm barrel and bolt carrier group kit gives you the core components to build a compact AR pistol or short-barreled rifle in the modern 5.7x28mm cartridge. This kit pairs CMMG's 16.1-inch barrel with a matched bolt carrier group engineered around the company's proprietary Radial Delayed Blowback system—a design that sets it apart from the lighter recoil impulse and reliability you'd expect from a purpose-built 5.7 AR platform.

What makes Radial Delayed Blowback relevant to you: compared to straight blowback systems, it manages recoil signature and bolt carrier weight differently, while retaining functional features like last-round bolt hold open. If you're building a compact tactical rifle, a truck gun, or an AR pistol chambered in 5.7x28mm, this kit handles the heavy lifting. The barrel is ready to drop into a standard Mk4/mil-spec AR15 upper receiver; the BCG works with either a CMMG Mk57 lower receiver or a standard AR15-pattern lower when paired with CMMG 5.7 conversion magazines.

This is a barrel-and-bolt package, not a complete upper or firearm, so you'll supply the upper receiver, lower receiver, and furniture. 5.7x28mm ammo is compact, mild-shooting, and increasingly available—ideal if you want sub-rifle ballistics in a configurable platform.

Key Features

• 16.1-inch 5.7x28mm barrel for compact AR pistol or SBR builds
• CMMG Radial Delayed Blowback bolt carrier group—reduces recoil and manages carrier weight versus direct blowback
• Last-round bolt hold open functionality
• Compatible with Mk4/mil-spec AR15 uppers
• Works with CMMG Mk57 lowers or standard AR15 lowers using CMMG 5.7 conversion magazines
• Black finish

Specifications

Frequently Asked Questions

What lower receivers does this barrel and BCG kit work with?

The kit is compatible with a CMMG Mk57 lower receiver. It also works with a standard Mk4/AR15 mil-spec lower receiver when you use CMMG 5.7 AR conversion magazines.

Is this a complete upper or just a barrel and bolt carrier group?

This kit contains the barrel and bolt carrier group only. You will need to source a Mk4/mil-spec AR15 upper receiver to complete the upper assembly.

What makes Radial Delayed Blowback different from standard blowback?

CMMG's Radial Delayed Blowback system manages recoil impulse and bolt carrier weight differently than a direct blowback design, resulting in less felt recoil while maintaining features like last-round bolt hold open.

What is 5.7x28mm good for?

5.7x28mm is a compact, low-recoil cartridge that delivers sub-rifle ballistics. It's commonly used in AR pistols and short-barreled rifles for tactical applications, training, and shooters who prefer mild recoil with a standard AR-platform ergonomic.