Originally Posted by
556Cliff
What Eric D. said, it's pretty simple to understand really... The threads on the barrel nut are usually seized to the threads on the upper, the inside clamp face of the barrel nut and the upper receiver extension flange clamp face don't usually become seized to the barrel extension flange.
This is why/how you can shear an index pin by using upper receiver vise blocks to remove/install a muzzle devise even though the barrel nut was properly torqued to spec. The friction clamping the flange is less than the friction between the threads of the barrel nut and upper receiver.