Cmmg Resolute Mk4 300Blk Assembl Upper 14.5''Bbl W/P&W Brake Blk

The CMMG Resolute Mk4 300BLK assembled upper receiver is built for shooters who need a genuine short-barrel setup without the paperwork. At 14.5 inches with a pinned and welded muzzle brake, this upper meets federal overall length requirements for an unserialized rifle upper—no SBR registration, no NFA stamps. It's a straightforward, go-to-work design: direct impingement operation, 1:7 twist barrel optimized for 300BLK ballistics, and a non-adjustable mid-length gas block that keeps things reliable and simple.

This is an assembled upper receiver only, which means you'll need to source your own bolt carrier group and charging handle to complete your build. The 4140CM barrel is finished in Cerakote, and the billet 6061-T6 aluminum receiver is paired with CMMG's EML13.5 handguard using M-LOK slots for accessories. The included CMMG ZEROED SVD muzzle brake is factory pinned and welded to the barrel—no guesswork, no timing issues.

The Resolute platform has earned a reputation for no-nonsense reliability. If you're building a short 300BLK rig for hunting, home defense, or suppressed shooting, this upper gives you the barrel length you want while staying legal as an unserialized receiver assembly. Weighs 4.1 lbs with a total length of 23.8 inches, so it'll pair well with a quality lower to create a genuinely compact, maneuverable rifle.

Key Features

• 14.5" 300BLK barrel with 1:7 twist—optimized for subsonic and supersonic loads
• Pinned and welded SVD muzzle brake meets federal length requirements without NFA registration
• Direct impingement, mid-length gas system with fixed gas block
• Billet aluminum receiver with integrated Picatinny rail
• CMMG EML13.5 M-LOK handguard for tactical flexibility
• Cerakote finish for durability and corrosion resistance
• Assembled upper—BCG and charging handle sold separately

Specifications

Frequently Asked Questions

Do I need an SBR stamp or Form 1 for this upper receiver?

No. The 14.5" barrel with the pinned and welded muzzle brake is measured as a complete unit. The total length of 23.8" meets federal rifle length requirements, so the upper receiver itself is not regulated as an SBR. Pair it with any standard lower receiver and you have a legal rifle without NFA paperwork. Always confirm your state and local laws, but federally this setup avoids the SBR registration process.

What BCG and charging handle do I need?

This is an assembled upper receiver only. You'll need to purchase a bolt carrier group and charging handle separately. Both must be chambered in 300BLK (or larger calibers that support the platform) and compatible with AR-15/Mk4 uppers. Any standard mil-spec or commercial BCG and handle will work with this receiver.

Is the muzzle brake removable?

No. The CMMG ZEROED SVD brake is pinned and welded to the barrel at the factory. It is not designed to be removed or replaced without machining work. This is intentional—the pinned and welded configuration is what allows the 14.5" barrel to legally meet rifle length requirements without an SBR.

What's the 1:7 barrel twist good for?

A 1:7 twist stabilizes a wide range of 300BLK ammunition, including heavy subsonic rounds (200+ grains) and standard supersonic loads. This versatility is one reason the 1:7 twist is the standard for 300BLK rifles.