If you have an ar-9, you want this. - OpticsPlanet Exclusive Maxim Defense Industries RDB Roller Delayed Blowback System

Installed on my suppressed Mk2 9mm from griffin. The recoil impulse went from making my collar bone ache after a few hundred rounds with a pistol brace to a light tap that doesnt take me off target at all. It doesn't quite reach rimfire territory, but it's so very close. Charging is more difficult, but still can be done quickly with 1 hand and a normal ambi handle even with my unimpressive upper body strength. I can now run the trigger as fast as my fingers will allow without coming off target until the massive cloud of gas makes my eyes burn. The gas to the face IS lessened, I no longer have to stop partway through a 32 round mag to blink away tears, but it's not a perfect cure by any metric when running suppressed. If you have a heavy bolt weight and buffer, expect a noticeably lighter gun as well.