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 5.7×28 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 six 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 – 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 – Muzzle Device: CMMG ZEROED Linear Compensator – Muzzle Thread: 1/2 x 28 – 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