I thought this was gonna be about making it full auto, I was ready to be all, "super illegal bro." Other than that sorry I can't help but good luck.
I'm gonna stop you right there, you just said an AR-15 is a high-powered rifle. It is obvious you don't know anything about guns and I shall ignore anything you say from this point on.
922r is a must from what I've looked into. What I'm wrestling with right now is the price. It's looking like it would be in the $3-4K range. That kind of money would buy a MAC. I know the quality is much better on the UMP conversion, but it will be a "neutered" PDW. I'm still trying to get as much user experience info as possible, but not many people seem to know a whole lot about this conversion.