The typical homebrew answer to unscrewing barrels goes like this:
1) Pull off the cylinder *completely*.
2) Pad a vice with wood. Grab barrel in same.
3) Carve the end of a 2x4 so it fits the frame in the hole where the cylinder was *perfectly* or close to it.
4) You DID make sure the 2x4 was at least 3ft long and 5ft is better?
5) Use the 2x4 as a "wrench", twisting the frame off the barrel.
If you're not a gunsmith, or don't at least have a good gunsmithing manual for that gun, you really ought to think twice about this. Stainless steel can gall and you can screw up the frame if you don't know what you're doing. And then when you put that barrel or another back on, things get really interesting: is it on tight enough yet still has the right gap? And where is the front sight when you get to that point?! Getting all three right at once is so hairy most gunsmiths don't bother: they either remove and relocate the front sight, or if the place where the front sight was is now too ugly, chop the barrel back 3/4", recrown and mount the front sight on clean metal.