CMMG AR-15 Resolute 300 MkGs 9mm Upper Receiver Assembly - 16.1" Barrel, Radial Delayed Blowback, M-LOK Handguard

The CMMG Resolute 300 MkGs is a complete 9mm upper receiver assembly purpose-built for shooters who want a soft-shooting, reliable pistol caliber carbine without the complexity of piecing together incompatible parts. Built on CMMG's proven radial delayed blowback operating system, this 16.1-inch barreled upper delivers the accuracy and control you'd expect from a quality PCC while maintaining the ergonomics and modularity of the modern AR platform. Whether you're running steel through a suppressor at the range, setting up a dedicated home-defense carbine, or just want the smoothest-cycling 9mm AR you can find, the Resolute 300 MkGs does the heavy lifting so you don't have to.

What sets this upper apart is thoughtful engineering paired with what you actually need out of the box. The radial delayed blowback system is the reason this gun shoots so soft—there's no carrier tilt, less wear on components, and genuinely pleasant recoil impulse even with full-power loads. The 16.1-inch 4140CM barrel is twist-rated 1:10 for stable, consistent ballistics, while the billet aluminum upper and M-LOK handguard give you a platform that's both rigid and simple to accessorize. Everything ships assembled and tested: bolt carrier group, ZEROED ambi charging handle, ZEROED muzzle brake, and polymer dust cover. You're not buying promises; you're buying a working gun.

Platform compatibility is straightforward. This upper pairs with standard AR-15 lowers when fed 9mm conversion magazines, or with CMMG's pistol-mag lowers (Mk9, MkGs, Mk17) for direct feed reliability. That flexibility means you can build exactly what your setup demands. The Cerakote finish comes in multiple colors, so durability and aesthetics aren't an either-or choice.

Key Features

• Radial delayed blowback operating system for soft recoil impulse and reliable cycling
• 16.1-inch 4140CM barrel with 1:10 twist for consistent 9mm ballistics
• Billet 6061-T6 aluminum upper receiver, rigid and corrosion-resistant
• CMMG EML15 M-LOK handguard for easy accessory mounting
• Fully assembled with ZEROED bolt carrier group, charging handle, muzzle brake, and dust cover
• Designed for use with standard AR-15 lowers (with 9mm conversion mags) or CMMG pistol-mag lowers
• Professional Cerakote finish available in multiple colors

Specifications

Frequently Asked Questions

What lower receivers are compatible with the Resolute 300 MkGs upper?

This upper works with standard AR-15 lowers when you use 9mm conversion magazines, or with CMMG's dedicated pistol-magazine lowers (Mk9, MkGs, and Mk17 models). That gives you flexibility to build around whatever lower you already have or prefer.

Is this upper fully assembled, or do I need to finish the build?

The upper comes complete and ready to mount on a compatible lower. The bolt carrier group, charging handle, muzzle brake, and dust cover are all included, assembled, and tested by CMMG.

What makes the radial delayed blowback system different?

Unlike direct impingement or simple blowback systems, radial delayed blowback reduces bolt tilt and carrier stress, which results in a softer recoil impulse, smoother cycling, and longer component life—especially noticeable in 9mm carbines where felt recoil is already mild.

Can I suppress the Resolute 300 MkGs?

Yes. The muzzle brake included on this upper has a 1/2 x 28 thread, which is a standard suppressor mounting thread. You can swap the brake for a threaded muzzle device or a suppressor mount.