I don't know much about AR-15's, and even less about combo machines. But I have threaded and chambered quite a few replacement barrels. The best luck I've had is with the barrel through the headstock on a fairly heavy lathe. What you don't want is chatter to get started, and since a chamber reamer is cutting along its entire length, it REALLY is prone to start chattering. So spiral fluted reamers, working with the chamber up close to the headstock, on a nice sturdy lathe, are all good things (in my book)
I'm here hoping to advancify my smartitude.