I pre-ordered a couple of these Endomag 9mm kits about 4 months ago. They allow you to use a standard P-Mag in an AR Lower. No need for a mag block. Received them last week and just tested them today. I tested them on 2 different semi lowers and on an M16 lower. Also used 2 different 9 mm uppers, both of which are integrally suppressed. The mags fed fine in both semi and full auto. The last test was a mag dump (loaded to 28 rounds) on full auto. The mag fed fine with no issues. This sure makes swapping uppers a much simpler task when you don't have to also use a mag block.