CMMG Mk4 5.7X28mm Resolute Upper Group Receiver Review

First Impressions of CMMG Mk4 5.7X28mm Resolute Upper Group Receiver

The CMMG Mk4 5.7X28mm Resolute Upper Group Receiver promises to bring the flat-shooting, low-recoil 5.7x28mm cartridge to the familiar AR-15 platform. CMMG has built a solid reputation for quality AR components and innovative designs, and this upper receiver group aims to deliver PDW performance in a carbine format. The allure of a lightweight, controllable rifle firing a high-velocity round is hard to resist, especially for those looking for something different from the standard 5.56 NATO.

My interest in the CMMG Mk4 5.7X28mm Resolute Upper Group Receiver stemmed from a desire to explore alternative personal defense calibers without completely abandoning the AR platform I know so well. I’ve always been intrigued by the 5.7x28mm’s ballistics and its potential for home defense and varmint hunting, but I hesitated to invest in a dedicated 5.7x28mm firearm. The CMMG upper offered a chance to dip my toes in the water without a full commitment.

Upon unboxing, the CMMG Mk4 Resolute Upper Group Receiver exuded quality. The finish was clean and consistent, the M-LOK handguard felt solid, and the overall fit and finish appeared top-notch. It felt lighter than my standard 5.56 upper, a promising sign for maneuverability.

Compared to complete 5.7x28mm AR-style rifles from other manufacturers, like the Ruger 57 or the FN Five-seveN pistol, the CMMG upper offered a more modular and customizable experience. I chose it because it allowed me to utilize my existing AR lower, saving money and maintaining familiarity. My first impression was definitely positive; I was excited to get it to the range.

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Real-World Testing: Putting CMMG Mk4 5.7X28mm Resolute Upper Group Receiver to the Test

First Use Experience

My first range session with the CMMG Mk4 5.7X28mm Resolute Upper Group Receiver took place at an outdoor range on a moderately warm, sunny day. I paired it with my trusted AR-15 lower receiver and CMMG’s 5.7 AR Conversion Magazines. I tested various 5.7x28mm loads, including Federal American Eagle and FN SS197SR.

The upper performed remarkably well right out of the box. The recoil was noticeably lighter than a 5.56 AR, making it easy to stay on target for rapid follow-up shots. I experienced zero malfunctions during the first 100 rounds. The upper was easy to control, and the muzzle brake effectively reduced muzzle rise.

Extended Use & Reliability

After several months of use, the CMMG Mk4 Resolute Upper Group Receiver continues to impress. It has held up well to regular range trips and a couple of varmint hunting excursions. I’ve put approximately 500 rounds through it with only one minor hiccup (a failure to feed that was likely magazine-related).

Durability has been excellent; there are no visible signs of excessive wear or tear. Cleaning and maintenance are straightforward, similar to any other AR upper. The bore cleans easily, and the bolt carrier group requires standard lubrication. Compared to my previous experiences with other AR platforms, the CMMG upper provides a unique shooting experience due to the 5.7x28mm cartridge’s flat trajectory and minimal recoil.

Breaking Down the Features of CMMG Mk4 5.7X28mm Resolute Upper Group Receiver

Specifications

The CMMG Mk4 5.7X28mm Resolute Upper Group Receiver boasts the following specifications:

• Caliber: 5.7x28mm – This caliber offers a flat trajectory and low recoil, making it ideal for accurate shooting at medium ranges.
• Overall Length: 25.3 inches – This length provides a good balance between maneuverability and ballistic performance.
• Barrel Length: 16.1 inches – The 16.1-inch barrel strikes a balance between velocity and compactness, making it suitable for various applications.
• Barrel Profile: Medium Tapered – This profile helps to reduce weight while maintaining accuracy.
• Barrel Twist Rate: 1-9 – The 1-9 twist rate is optimized for stabilizing common 5.7x28mm bullet weights.
• Threads per Inch: 1/2×28 – This is a standard thread pitch, allowing for compatibility with a wide range of muzzle devices.
• Muzzle Device: CMMG Zeroed Muzzle Brake – The included muzzle brake helps to reduce recoil and muzzle rise.
• Fabric/Material: 6061-T6 Aluminum – The use of 6061-T6 aluminum provides a lightweight yet durable construction.
• Gun Rail: M-LOK – The M-LOK handguard allows for easy attachment of accessories.
• Gun Make: CMMG – This indicates the manufacturer of the upper receiver group.
• Gun Model: CMMG Mk4 – This denotes the specific model of the upper receiver group.
• Weight: 4 lb – The weight is relatively light, enhancing maneuverability.
• Magazine Included: No – Magazines must be purchased separately, and CMMG recommends their 5.7 AR Conversion Magazines.
• Mount Type: M-LOK – This refers to the mounting system used on the handguard.

Performance & Functionality

The CMMG Mk4 5.7X28mm Resolute Upper Group Receiver performs admirably, delivering consistent accuracy and reliable function. The flat trajectory of the 5.7x28mm cartridge makes it easy to engage targets at distances where a 5.56 would require more significant holdover. Its low recoil makes it very controllable and allows for quick follow-up shots.

The primary weakness is the reliance on CMMG’s proprietary 5.7 AR Conversion Magazines. While they function well, they are relatively expensive and not as widely available as standard AR-15 magazines. Despite that, it does meet and exceed my expectations.

Design & Ergonomics

The CMMG Mk4 boasts a well-thought-out design. The M-LOK handguard offers ample space for mounting accessories, and the overall weight is manageable. The fit and finish are excellent, as expected from CMMG. The controls are standard AR-15 fare, so there’s no learning curve for experienced AR users.

Durability & Maintenance

The 6061-T6 aluminum construction suggests good durability. I anticipate this upper receiver group lasting for many years with proper care and maintenance. Cleaning is simple and straightforward, and replacement parts (if ever needed) should be readily available from CMMG.

Accessories and Customization Options

The CMMG Mk4 5.7X28mm Resolute Upper Group Receiver readily accepts standard AR-15 accessories, such as optics, lights, and slings. The M-LOK handguard allows for the attachment of vertical grips, angled foregrips, and other M-LOK compatible accessories. The 1/2×28 threaded barrel allows for the use of aftermarket muzzle devices, suppressors, or flash hiders.

Pros and Cons of CMMG Mk4 5.7X28mm Resolute Upper Group Receiver