I don't remember where I read it, but somebody did some pretty good tests of all the penetrating oils out there. The best one was a homemade 50/50 mix of acetone and automatic transmission fluid.
I would stand the barrel muzzle up, pour a couple of ounces of that mix down, plug the muzzle with a dowel, and let it sit for a couple of days in that position.
Well you are waiting for it to soak, get a bigger vice and lag bolt your bench to the wall studs.