I don't know much about barrel profiles, but I measured it.
The barrel in front of the gas block measures 0.722". Under the hadnguards it is tapered, but at its narrowest point it measures 0.606".
The muzzle brake isn't "permanently affixed". It is pinned on with two pins. I called Armalite a while back and they advised me that the barrel itself is knurled, then the brake is pressed on and pinned. There is supposed to be plent of barrel uderneat to turn it down and thread it for a flash hider.