Yeah, Jon, I just got a chance to get my hands on the AR-15 to see the limitations for myself. I was surprised but hey, it was designed for the 5.56 and that is what fits, huh.....lol Now I guess I need to look at the AR-10 and see if I can make it work... I don't mind re-machining a few things and obviously a new barrel. There is just no way the AR-15 platform will be enough for what I want to do. I realize many people do not understand slugging heavy ammo but when you only have limited velocity (subsonic), I want too see some of that energy back and that takes a serious slug.