The movement issue is all speculation as of right now. I'll wait to see what the 2 in this thread say, who actually have one and fire it (Moe; once he get's it, and sodak; once he fires it after tightening it.)
As for the open end? That's a non issue, IMO. How many free-float AR rails are open ended? Most, if not all (you can buy "end caps" for YHM rails, though). No problems there.
Just one more thing to mention, if any of you guys are waiting for me to fire it and see the results, you better not be in any hurry. I'm saving my ammo for a shoot on the 26th of Sepember so I won'