A round barrel vise with aluminum shims and a bit of pine rosin makes the job much easier. If the breech-tang is tapered, a breech plug wrench is a better option than an adjustable (crescent) wrench as the adjustable wrench will deform the upper surface of the tang. On stubborn, tapered-tang plugs, I would make a "clamp" from two pieces of solid steel and two bolts, then engage the clamped plug with a 4 foot pipe wrench. A week soaking the breech in a can of Kroil, or kerosene-ATF is often a big help. A 2 pound rubber mallet to provide a bit of "shock and awe" will sometimes be needed to get the process moving. Leverage is your friend! I have successfully pulled the breech plugs on over 100 original muskets, both flintlock and percussion, one of which had 7 loads superimposed in it. If you don't need to pull it, breech plug are best left alone.