I make a lot of bbl vise 'jaws' for cartridge guns I work on.
Most I make by casting them from lead right from the bbl itself as the form. A simple sheet metal box with a cut out at each end to lay the bbl into, some shims to level the bbl a bit to 1/2 depth and pour 2 jaws.
If the jaws bottom out before they clamp down securely, shim one or both of the channels under the bbl itself with a cardboard shim to raise the bbl up. Rosin (or suger!) helps.
I use a' bbl vise', but as I'm usually unscrewing. re-assembling a recv'r to a bbl,,but a breech plug can be done in a stout bench vise generally.
For that and a round bbl, you can take a square piece of maple and drill a round hole through the middle big enough so the bbl drops through to the depth you want to secure it. The other end will now be loose because of the bbl taper.
Take the bbl and coat it with release agent (paste wax/auto shine-ola wax works good). Mix up some of your favorite epoxy .
Drop the bbl back into the wooden block and tap it in place. Then fill the other end void up with the epoxy keeping the bbl centered in the drilled hole. Let it cure.
Knock the block forward off the bbl.
Then with a saw,,hand or table,,saw a fairly wide saw cut kerf through one side of the block and into the cavity. Don't cut the block in half. Just a clearance cut through one side so the block can flex and hinge when clamped.
Clean any wax from the bbl and the block. Coat with rosin, and slide the bbl back into the block.
Clamp the assembly inito the vise and tighten with the saw cut upwards in the vise. You'll see it close up as you tighten the vise.
Remove the breech plug.
If the bbl starts to turn in the block and you need to tighten more but the block is bottomed out, take the bbl out and make the saw cut kerf wider to allow more clamping power.
I like the one piece block. You never loose the other half of the jaw set and never have to match up set halves when you end up with dozens of them around the shop,,if you get to that point!