I use a barrel vise and wrap two layers of file folder cut as long as the vise, or just a little longer. I put the barrel in the vice with the paper wrapped around the barrel at the thickest point determined with a caliper. tighten the vise nuts evenly not using too much force. They need to be tight, but over tightened.
Using a 2 to 3 pound hammer, I pop the barrel nut wrench a few times (after removing the front scope nut) and the barrel nut has loosened every time.
Many use two pieces of 2x4's in a vice. Either way, I have never damaged a barrel, but I always use common sense when removing one.
Dennis