I've had two of the mec tech glock 17 "uppers." They would function 99% with the factory mags; you just never knew when the other 1% would appear. Tried the full range of ammo, 110-147 grain bullets slow, medium and fast. When they worked they were fun but felt somewhat awkward in handling. The thing you will probably notice most is the bolt movement. As a pure blowback there is a lot of metal sliding back and forth in the tube, rather like a Smith M76 or swedish K. The 9mm AR's seem a bit smoother, possibly due to the in line stock, and of course you can trick the AR's out to the limit of your wallet. A rifle in 9mm is mostly a fun plinker anyhow (Not that there's anything wrong with that...).