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AR Parts Notice - State/local restrictions may apply in CT, DC, IL, MA, NJ, NY, or WA. Customers must review our Shipping Policy prior to ordering.


Gaining more acceptance as the go to personal defense caliber for many shooters in recent years, 5.7x28mm has proven itself to be well suited for the job. With incredibly flat shooting performance and low recoil, 5.7x28 excels at closer ranges while still providing ideal penetration. Equipped with a 5” barrel, this is the most compact 5.7x28mm upper group CMMG® offers. CMMG 5.7x28mm uppers are designed to work with CMMG Mk57™ lowers or standard AR15/Mk4 lowers (when used with 5.7 AR Conversion Mags). Available in different Cerakote finishes, all CMMG® Upper Groups come complete with bolt carrier group and ZEROED™ accessories including:

ZEROED AR15 Polymer Dust Cover

ZEROED AR15 Ambi Charging Handle

ZEROED Linear Compensator 5.56mm

Radial Delayed Blowback™ Operating System

Originally developed for the .45 ACP MkG platform, CMMG has adapted our Radial Delayed Blowback system across our pistol caliber lineup to create pistols and rifles with a softer recoil impulse and reduced bolt weight compared to traditional straight blowback systems. Check out our CMMG Resources article & video to learn more about what sets Radial Delayed Blowback apart.


Lifetime Quality Guarantee

We aim to please, that’s why every CMMG product is backed by our Lifetime Quality Guarantee against material and workmanship defects. Learn More.

Technical Specifications

Platform Mk4, Mk57
Caliber 5.7x28mm
Operating System Radial Delayed Blowback
Barrel Length 5
Barrel Twist 1:9
Barrel Contour MT
Barrel Material 4140CM, SBN
Barrel Twist 1:9
Muzzle Device CMMG ZEROED Linear Compensator
Muzzle Thread 1/2 x 28
Gas Port Location N/A
Gas Block N/A
Upper Receiver Billet 6061 T6 Aluminum
Hand Guard CMMG EML4 M-LOK
Finish Cerakote Standard
Charging Handle CMMG ZEROED Ambi
Weight 2.5 lbs
Length 13.8"


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