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 Uppers are designed to work with any standard AR15/Mk4 lower and 9mm AR Conversion Mags or 9mm pistol magazine lower platforms: Mk9 (Colt-pattern mags), MkGs (Glock-pattern mags), Mk17 (Sig P320 mags).