Mean Arms has developed probably the most refined and sophisticated 9mm AR delaying system on the market, what they call Bearing Delay. This began as an exploration into fitting HK-style roller delay into and AR, and ended up with a system using three spherical bearings to redirect chamber pressure to delay opening. The system is particularly well suited to high end competition use, where the development data form Mean Arms can be used in conjunction with different "lifters" (the equivalent of the locking wedge in a roller delay system), buffers, and ammunition types to tune the system for minimal muzzle climb and felt recoil.

Disclaimer: Mean Arms sent me this upper for filming. The lower assembly I provided myself.