CMMG RESOLUTE MK4 Upper — 5.56x45mm NATO, 16.10" Barrel, Armor Black, M-LOK Free-Float Handguard (No BCG/Charging Handle) — 55BB40BAB

Give your next AR build a premium foundation with the CMMG RESOLUTE MK4 upper in 5.56x45mm NATO. Featuring a versatile 16.10" barrel, a slim M-LOK® free-float handguard, and a durable Armor Black finish, this factory-assembled upper is engineered for reliability, balance, and accessory flexibility. It ships without a bolt carrier group (BCG) or charging handle, letting you select your preferred components for the exact feel and service life you want. From practice and training to duty-ready carbines, the RESOLUTE MK4 delivers the accuracy and consistency serious shooters expect from CMMG.

Why This Upper

• CMMG precision: Clean receiver/barrel fitment and consistent gas tuning for dependable cycling.

• 5.56 NATO chamber: Run both 5.56x45mm and .223 Rem ammunition for maximum range-day flexibility.

• 16.10" length: Smart balance of velocity, maneuverability, and legal simplicity—ideal for general-purpose carbines.

• Free-float M-LOK handguard: Improves repeatable accuracy and gives you modular mounting for lights, grips, bipods, and QD sling cups.

• Armor Black finish: Low-glare, abrasion-resistant coating that stands up to hard use and harsh weather.

• Mod-your-way: Ships sans BCG and charging handle so you can tailor coatings, weight, and ergonomics.

Key Features at a Glance

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

• Barrel Length: 16.10"

• Handguard: Free-float M-LOK (slim profile)

• Receiver/Handguard Finish: Armor Black

• Included: Complete upper w/ barrel, gas system, handguard, and muzzle device (config may vary by lot)

• Not Included: BCG and charging handle

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

• SKU / Model: 55BB40BAB

• UPC: 810103473461

Spec notes: Typical AR-15 features for this category include 1/2"-28 muzzle threads, .750" gas block journal, and a fast twist rate for 55–77gr bullets. Confirm exact specs on your unit against your build plan.

Performance You Can Feel

The MK4’s free-float design keeps handguard pressure off the barrel for more consistent groups, while CMMG’s gas system tuning supports smooth function with common training and defensive loads. Combined with your chosen BCG and charging handle, you’ll get a carbine that points quickly, shoots flat, and holds zero—range after range.

Build It Your Way

• Choose the BCG: Phosphate for duty-proven durability, nitride for easy cleaning, or DLC/NP3 for slick cycling.

• Pick your handle: Standard, extended-latch, or ambidextrous for better manipulations.

• Tune the end: Add a flash hider, compensator, brake, or suppressor mount (where legal) on standard threads.

• Mount accessories: Use the top Picatinny for optics and M-LOK slots for mission-driven accessories without excess weight.

Ideal For

• All-purpose AR builds balancing home/ranch defense and training.

• Competition practice where free-float accuracy and fast handling matter.

• Custom projects requiring a high-quality upper and user-selected BCG/CH.

• Agency/Range carbines standardized around 5.56 NATO.

Installation Tips

1. Headspace check: Verify headspace with your chosen BCG/bolt before live fire.

2. Muzzle device timing: Use a new crush washer or shims and torque to spec.

3. Gas/buffer tuning: Pair with appropriate buffer/spring for your ammo and suppressor use (if applicable).

4. Fastener discipline: Torque and thread-lock rail and accessory screws per manufacturer guidance.

Care & Maintenance

• Break-in: Modern barrels often require minimal break-in—prioritize quality lube and consistent cleaning.

• Routine: Keep the chamber, lugs, gas system, and rail fasteners clean and inspected.

• Finish care: Wipe Armor Black surfaces with a light oil film after exposure to rain, sweat, or salt air.

Specifications (Summary)

• Brand: CMMG

• Series: RESOLUTE MK4

• Caliber: 5.56x45mm NATO

• Barrel: 16.10"

• Handguard: M-LOK free-float

• Finish: Armor Black

• Includes: Upper assembly (barrel, gas system, handguard, muzzle device)

• Excludes: BCG & charging handle

• Model/SKU: 55BB40BAB

• UPC: 810103473461

SEO-Optimized Highlights

• CMMG RESOLUTE MK4 5.56 upper, 16.10", Armor Black

• M-LOK free-float handguard for accuracy and accessory mounting

• Ships without BCG or charging handle for custom builds

• 55BB40BAB, UPC 810103473461

• Premium AR-platform upper for training, defense, and competition practice

Suggested Meta Title

CMMG RESOLUTE MK4 5.56 Upper — 16.10", Armor Black, M-LOK Free-Float (No BCG/CH) | 55BB40BAB

Suggested Meta Description

Build smarter with the CMMG RESOLUTE MK4 5.56x45mm NATO upper. 16.10" barrel, Armor Black finish, M-LOK free-float handguard, ships without BCG/charging handle for custom setups. SKU 55BB40BAB, UPC 810103473461.

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“CMMG RESOLUTE MK4 5.56x45mm NATO upper, 16.10-inch barrel, Armor Black with M-LOK handguard (55BB40BAB)”

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

Armor Black durability, free-float accuracy, and CMMG reliability—the RESOLUTE MK4 upper is a rock-solid core for your next AR build. The End