Two thoughts on the above.
First, the chair vice - seems like some of us would be able to apply more holding force than others and, in my case, I'm trying to have less rather than more force in that area.
Second, the concern about applying too much force in a small area. At first thought it would seem that the pressure inside the barrel when shooting would be way more force than a hand held pair of vice grips could generate. My concern would be grunging up the barrel if the jaws weren't padded with the right stuff or allowed the barrel to rotate.
I thought about basically holding the rifle by clamping onto the gas block with the padded jaws and then wrenching the fh off. On second thought I was worried that the barrel would rotate within the gas block. On third thought, I wondered if the barrel retaining bolt would keep the barrel from rotating. Then on fourth thought I was worried the retaining bolt would allow the barrel to rotate and grunge up the barrel or bolt or both.
The sitting on it idea appears to be a variant of the idea of keeping the barrel from rotating by holding the gas block. But I'm going to try just holding it by hand and applying the wrench first after making sure the barrel retaining bolt is tight.
Any other ideas?