The barrel is not part of the threaded system. Just like turning on the FH with only the upper secured, turning on the nut with only the barrel secured could cause you to enlarge the index pin notch.
It's just the right way to separate two threaded halves: hold one half, turn the other. In the case of the barrel nut and the upper receiver the barrel itself is not one of those halves.
Or, do it your own way. It's been awhile since we had one of those "hey, I tried to build a gun with a bent butter knife and a 2x4 and now it's ****ed up" kinds of threads.
:D

