CMMG 55BB40BAB Resolute MK4 5.56x45mm NATO 16.10", Armor Black, M-LOK Free-Float Handguard for AR-Platform (No BCG or Charging Handle)

The CMMG Resolute MK4 5.56x45mm NATO upper receiver is purpose-built for AR platform builders who want a no-frills, straightforward upper that gets to work without breaking the bank. This 16.1" assembled upper delivers the ballistic performance and ergonomic foundation hunters, sport shooters, and defensive rifle builders expect—and at 3.6 lbs, it won't bog down your finished gun. The billet 6061 T6 aluminum upper receiver pairs with a free-floating M-LOK handguard, giving you solid mechanical stability and modern rail real estate for optics, lights, and accessories. Whether you're completing a first build or adding to an existing platform, this upper handles the job with the kind of proven reliability CMMG is known for.

Under the hood, the Resolute MK4 runs a direct impingement system with a 4140CM steel barrel (1:7 twist, mid-length gas), non-adjustable 0.750" gas block, and a standard CMMG A2 flash hider threaded 1/2 x 28. That 1:7 twist stabilizes a wide range of 5.56 ammunition, from 55-grain practice loads to 77-grain match and hunting rounds. The setup is straightforward—no tuning required out of the box, though you'll need to source your own bolt carrier group and charging handle to complete the upper.

The M-LOK free-float handguard frees the barrel from pressure points, supporting accuracy and consistency shot after shot. At 25" overall length, it's compact enough for confined spaces or vehicle storage but still gives you a proper sight radius and enough rail space to mount whatever optic or accessory setup your mission calls for. This is a versatile upper for hunting, three-gun competition, home defense, or range duty.

Key Features

• Billet 6061 T6 aluminum upper receiver for durability and precision
• Direct impingement system with proven reliability
• 16.1" 4140CM steel barrel, 1:7 twist for broad ammunition compatibility
• Free-floating M-LOK handguard for accuracy and modern accessory mounting
• CMMG A2 flash hider (1/2 x 28 threaded) included
• Non-adjustable mid-length gas block
• Compact overall length (25") and lightweight (3.6 lbs)
• Armor Black Cerakote finish

Specifications

Frequently Asked Questions

Does this upper receiver come with a bolt carrier group and charging handle?

No. This is an upper receiver assembly only—BCG and charging handle are not included. You'll need to source these components separately to complete your build.

What ammunition does the 1:7 barrel twist stabilize?

The 1:7 twist barrel is versatile across the 5.56 spectrum, from lightweight 55-grain practice ammunition through mid-range target loads and up to 77-grain match or hunting rounds. This twist rate is a safe, all-purpose choice for most shooters.

Is the handguard really free-floating?

Yes. The M-LOK handguard is free-floating, meaning it does not make contact with the barrel. This design supports consistent accuracy by eliminating pressure points and harmonic inconsistencies.

What platforms and lower receivers does this upper work with?

This upper is compatible with standard AR-platform lower receivers configured for 5.56x45mm NATO. Compatibility depends on your lower receiver being spec-correct; we recommend consulting your lower receiver manufacturer or a qualified gunsmith if you have questions about fit.