The slick and easy trick for nearly 100percent contact on your barrel-vise jaws for a pair of SxS (or drilling/vierling/funfling.../) tubes is to use Bondo or similar polyester resin filler material (as used in autobody panel repair etc)
Wrap your barrels in thin plastic-wrap (from the kitchen pantry) and set them a pair of wood (or whatever material) vise jaw blanks set-up with the two-part polyester filler material.
It sets up very quickly, and once hardened will give you a perfectly sure-footed grip on your barrel set.
cheers
Tinker
