I've found the barrel can be held quite positively with a quick wrap with glossy card stock and a layer of masking tape and a set of AN vise blocks http://www.summitracing.com/parts/aer-fcm3661 . Since the vise and the barrel are both steel and the vise blocks are aluminum, the aluminum will deform first but it never comes to that. Clamp close to the action so you don't bend the barrel and then an action wrench makes life easy.

EDIT: This requires a substantial vise, not some small wimpy job.