I use a big 6" machinist vise for barrel work, with 1/16" thick sheet brass covering the jaws so I don't put teeth marks in the barrel. Wood, leather, etc. is going to be too flexible and allow the barrel to twist out of the jaws as you remove the plug. If it's a tapered barrel, take up some of the taper with some extra shims of thin brass or steel, and remember to grip the barrel just ahead of the breech plug threads.
-Eric