I have a registered lower for my 5.56 LMT MRP CQB SBR (10.5" barrel). I assume it is legal to buy a 16" LMT 5.56 MRP barrel and use it on this rifle? By doing so, I would be leaving the 10.5" barrel in the safe; I don't have another 5.56 MRP upper, so it could only be used with the MRP upper which is attached to the registered lower. Is it legal to switch between barrel lengths (10.5" to 16" and vice-versa) on a registered SBR? Also, would it be legal to have more than one 10.5" barrel for one registered rifle? For instance one 10.5" barrel with a Noveske KX3 and another 10.5" barrel with a smaller flash hider? Thanks for any help that can be provided.