Radial delayed blowback was an advancement from straight blowback. Compact action radial delayed blowback was an advancement to the original radial delayed blowback. This (fixed ejector on radial delayed blowback) is a continued advancement bringing new technologies to the original platform. – If you’re looking to build the most compact 9mm AR then look no further. CMMG®’s 9mm radial delayed blowback uppers feature uncompromised design and engineering with styling that is never mistaken for something else. Having more options is always better like what magazines you would like to use. 9mm Radial Delayed Blowback uppers are designed exclusively to work with CMMG MkGs Mk17 or mil-spec AR15 lowers using our 9mm Conversion Magazines. Available in different Cerakote finishes all CMMG® Upper Groups come complete with bolt carrier group and ZEROED accessories including: – ZEROED Polymer Dust Cover – ZEROED AR15 Ambi Charging Handle – ZEROED Linear Compensator 9mm – Platform Mk4 Mk9 Mk17 MkGs – Caliber 9mm – Operating System Fixed Ejector Radial Delayed Blowback – Barrel Length 5 – Barrel Twist 1:10 – Barrel Contour MT – Barrel Material 4140CM SBN – Barrel Twist 1:10 – 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 2.6 lbs – Length 13.7