Be VERY careful when applying torque to the barrel while holding the upper in a vise or bench block. The alignment pin on the barrel extension is the ONLY radial link between the barrel and upper receiver. Applying excessive radial torque to the muzzle device can and will damage the upper receiver and/or alignment pin leaving those parts a total loss.
In extreme cases, if there's not sufficient space to clamp a barrel vise between gas block/FSB and muzzle, the gas tube can be removed by driving out the tiny pin holding it in the gas block and sliding further into the upper which will allow the use of a normal barrel vise on the barrel between gas block and barrel nut.