If you actually go ahead with our Masterkey application on your HBAR, I advise you to have the barrel turned at the appropriate location for the M203 QD latch in a way identical to a mil-spec M4 barrel. If not, you are going to need our Barrel Stop P/N: 97162 (and turn the barrel anyway, but in a different way) that is normally provided in the G.I. M5 RAS Pack-Out, not in the commercial m5 RAS package.
That being said, I agree with Troy. I don't think you will like the weight and bulk this adds to a rifle. I have tried it and i don't like it. Seems to work ok on a carbine if you are a team breacher, that is then left outside covering the street, but not if you are going room to room. However, on full auto, you can put all 30 rounds in one burst right in somebody's face as the muzzle is so heavy it "just hangs there" during the burst. Of course, the target has to be holding pretty still for you to have enough time to bring this much mass to bear very quickly.