RDB was an advancement from straight blowback. Compact action RDB was an advancement to the original RDB. This (fixed ejector on RDB) is a continued advancement bringing new technologies to the original platform. If youre looking to build the most compact 9mm AR then look no further. CMMGs 9mm RDB 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, and Mk9 9mm lowers, or with mil-spec AR15 lowers using our 9mm Conversion Magazines. Specifications for CMMG 9mm 8in Banshee Fixed Ejector Upper Group Receiver: Manufacturer: CMMG Caliber: 9mm Luger Overall Length: 16.5 in Barrel Length: 8 in Barrel Twist Rate: 1-10 Threads per Inch: 1/2x28 Muzzle Device: CMMG Zeroed Muzzle Brake Finish: Cerakote Fabric/Material: Aluminum Gun Rail: M-LOK Weight: 3 lb Barrel Material: 4140 Chrome Moly Vanadium Magazine Included: No Package Contents: CMMG 9mm 8in Banshee Fixed Ejector Upper Group Receiver