CMMG 30BC40AAB Resolute MK4 300 Blackout 16.10", Armor Black, M-LOK Free-Float Handguard for AR-Platform (No BCG or Charging Handle)

The CMMG Resolute MK4 300 Blackout upper receiver is a complete, ready-to-mount assembled upper built for shooters who want to run subsonic or supersonic rounds through a compact, modern platform. Chambered in 300 Blackout with a 16.1-inch barrel, this upper is ideal for shorter-range hunting, home defense, or suppressed shooting where the cartridge's ballistics shine. It ships without a bolt carrier group or charging handle, so you'll need to source those separately—a common arrangement that lets you choose components that match your build philosophy.

The barrel is a 4140CM steel tube with a 1:7 twist rate suitable for 300 Blackout's range of bullet weights, fitted with CMMG's A2 flash hider (1/2 x 28 threads) and paired with a non-adjustable mid-length gas block for straightforward, maintenance-free operation. The billet 6061 T6 aluminum upper receiver provides solid lockup and durability, while the CMMG EML15 M-LOK handguard offers a free-float design—meaning it doesn't contact the barrel—for consistency shot to shot. That separation is a genuine advantage if you're chasing accuracy or planning to add optics and accessories.

At 3.6 pounds and 25 inches overall, this upper strikes a practical balance between handling and velocity. The Armor Black Cerakote finish resists wear and corrosion through hard use. Whether you're assembling a quiet 300 Blackout host, a subsonic hunting rig, or a full-power short-range carbine, the Resolute MK4 delivers CMMG's no-nonsense engineering in a configuration that's ready to go once you add your BCG, charging handle, and lower receiver.

Key Features

• Assembled upper receiver; requires separate BCG and charging handle
• 300 Blackout chambering with 1:7 twist barrel for subsonic and supersonic loads
• 16.1-inch barrel for compact handling without sacrificing velocity
• Free-float M-LOK handguard for consistent accuracy and accessory mounting
• Non-adjustable mid-length gas system for simplicity and reliability
• A2 flash hider with 1/2 x 28 muzzle threads
• Billet aluminum upper receiver for strength and precise tolerances
• Armor Black Cerakote finish for corrosion and wear resistance

Specifications

Frequently Asked Questions

What's not included in this upper receiver kit?

The BCG (bolt carrier group) and charging handle are not included. You'll need to purchase these separately and install them before the upper is ready to fire.

Can I use this 300 Blackout upper on my existing AR-15 lower?

Yes. This is a standard AR-platform upper receiver compatible with any AR-15 lower receiver. Simply remove your current upper and attach this one to your existing lower.

Is the handguard actually free-floating?

Yes. The CMMG EML15 M-LOK handguard is free-floating, meaning it does not contact the barrel. This design improves accuracy consistency and allows for easier cleaning and maintenance.

What ammunition does this barrel work best with?

The 1:7 twist barrel is designed to stabilize the broad range of 300 Blackout ammunition, including both subsonic and supersonic loads. Check your ammunition manufacturer's recommendations to ensure proper performance.