CMMG RESOLUTE MK4 Upper — 5.56x45mm NATO, 16.10" Barrel, Midnight Bronze, M-LOK Free-Float Handguard (No BCG/Charging Handle) — 55BB40BMB

Build a premium AR the smart way with the CMMG RESOLUTE MK4 upper in 5.56x45mm NATO. This factory-assembled upper receiver group blends CMMG’s proven accuracy and reliability with a distinctive Midnight Bronze finish and a lightweight M-LOK® free-float handguard. It ships without a bolt-carrier group or charging handle, giving you the freedom to drop in the components you prefer. If you’re upgrading a workhorse rifle, assembling a duty-ready carbine, or finishing a custom lower, this MK4 upper is the heart of a build you can trust.

Why This Upper Stands Out

• CMMG Quality: Precision machining, clean fitment, and a reputation for dependable function under hard use.

• 16.10" Barrel in 5.56 NATO: Versatile length for velocity, balance, and maneuverability—ideal for training, home/ranch defense, and practical carbine work.

• Free-Float M-LOK Handguard: Improves consistency by keeping pressure off the barrel; add lights, grips, and sling QD’s exactly where you need them.

• Midnight Bronze Finish: Durable, low-glare coating with a premium look that resists wear, sweat, and weather.

• Modular by Design: Ships without BCG or charging handle, so you can choose your materials, coatings, and gas key/staking preferences.

• AR-15 Compatibility: Designed for standard AR-platform lowers (Mil-Spec pattern).

Key Features at a Glance

• Caliber: 5.56x45mm NATO (safe for .223 Rem)

• Barrel Length: 16.10"

• Handguard: Free-float M-LOK (slim profile for accessory mounting and improved heat management)

• Receiver Finish: Midnight Bronze

• Included: Complete upper receiver group with barrel, gas system, handguard, and muzzle device (configuration may vary by lot)

• Not Included: BCG and charging handle (choose your favorites and drop them in)

• Platform: AR-15 / MK4 (standard Mil-Spec lower)

• SKU / Model: 55BB40BMB

• UPC: 810103473508

Fitment Notes: Typical specs include 1/2"-28 muzzle threads and a .750" gas block journal; verify your unit’s exact configuration (gas system length, twist rate, and muzzle device) against your build plan.

Performance You Can Feel

A quality upper should lock up consistently, cycle smoothly, and hold zero. The free-float handguard protects barrel harmonics for predictable accuracy, while modern gas system tuning (paired with the right buffer/spring and your chosen BCG) supports reliable function across a wide range of ammunition. The result is a carbine that points quickly, shoots flat, and keeps running.

Build It Your Way

Because the upper ships sans BCG/charging handle, you can tailor the recoil impulse and maintenance profile:

• Choose phosphate, nitride, or DLC BCG coatings based on your cleaning routine.

• Pick a standard or ambidextrous charging handle for your manipulations.

• Add a compensator, brake, or suppressor mount (where legal) using standard 1/2"-28 threads (verify on your unit).

• Mount optics to the monolithic top rail and keep accessories tight on the M-LOK slots for a clean, snag-free setup.

Ideal For

• Range & Training Carbines: Reliable performance with common 55–77 gr loads.

• Practical/Defensive Builds: Proven 5.56 NATO chambering with accessory flexibility.

• Competition Practice: Free-float accuracy and lightweight handling help speed transitions.

• Custom Projects: Premium upper foundation for your favorite lower, trigger, and stock.

Installation Tips

1. Headspace & Inspection: When you add your BCG, confirm proper headspace using gauges.

2. Muzzle Device: Use a new crush washer or shims to time devices correctly; torque to spec.

3. Gas & Buffer Tuning: Pair the gas system with an appropriate buffer/spring for your ammo and suppressor use (if applicable).

4. Loctite & Torque: Follow manufacturer torque values for handguard accessories and use threadlocker where recommended.

Care & Maintenance

• Break-In: Modern nitrided/chrome-lined barrels often require minimal break-in; focus on quality lubrication.

• Cleaning: Keep the gas system, chamber, and lugs clean; inspect screws and rail fasteners periodically.

• Finish Care: Wipe the Midnight Bronze exterior with a light oil to maintain color depth and corrosion resistance.

Specifications (Summary)

• Brand: CMMG

• Series: RESOLUTE MK4

• Caliber: 5.56x45mm NATO

• Barrel Length: 16.10"

• Handguard: Free-float M-LOK

• Finish: Midnight Bronze

• Includes: Upper w/ barrel, gas system, handguard, muzzle device (varies)

• Excludes: BCG & charging handle

• Model/SKU: 55BB40BMB

• UPC: 810103473508

SEO-Optimized Highlights

• CMMG RESOLUTE MK4 upper, 5.56x45mm NATO, 16.10" barrel

• Midnight Bronze AR-15 upper with M-LOK free-float handguard

• Ships without BCG or charging handle for custom builds

• 55BB40BMB, UPC 810103473508

• Premium AR-platform upper for accuracy, reliability, and modularity

Suggested Meta Title

CMMG RESOLUTE MK4 5.56 Upper — 16.10", Midnight Bronze, M-LOK Free-Float (No BCG/CH) | 55BB40BMB

Suggested Meta Description

Build smarter with the CMMG RESOLUTE MK4 5.56x45mm NATO upper. 16.10" barrel, Midnight Bronze finish, M-LOK free-float handguard, and modular setup with no BCG or charging handle. SKU 55BB40BMB, UPC 810103473508.

Suggested Image Alt Text

“CMMG RESOLUTE MK4 5.56x45mm NATO upper, 16.10-inch barrel, Midnight Bronze with M-LOK handguard (55BB40BMB)”

Safety & Compliance: Assemble on an unloaded firearm only. Follow all federal, state, and local laws. Verify torque specs, headspace, and function before defensive use.

Premium machining, a tough Midnight Bronze finish, and a free-float M-LOK handguard—CMMG’s RESOLUTE MK4 upper is a rock-solid foundation for your next AR build. The End