That's a good suggestion for making a barrel vice block, but if you are using only that for removing/installing a barrel, you are flirting with trouble. The barrel nut is threaded to the reciever; that is the part which needs to be held when you work with the barrel nut. The fact that you have done three doesn't lessen your chances of twisting the reciever at the locating pin slot on the next one. Think about it, the nut doesn't really attach to the barrel, it just draws the barrel into the reciever thru pressure on the extension flange. When you tighten/loosen the nut while holding the barrel, all the resulting torque is applied to the locating pin and the threaded portion of reciever it fits into. This is how most canted front sights occur, distortion of the slot in the reciever.