There are a number of things you can try at home without a fancy setup. It's either that or pay a gunsmith to do it.
First off, I'd try soaking in penetrating oil. If you can find liquid WD-40 rather than spray, it's worth a try. It's a good penetrant but doesn't always break up rust very fast. Use a stiff brush and brush away as much rust as you can, then let it soak some more. It'll come free sooner or later, but it takes a little time.
Here's my trick-Brownells Steel White will melt the rust (and the bluing) like magic. I've used it to restore BP guns that were literal rustballs. Again, it takes some time and work, but it basically dissolves the rust away. Follow the directions and it won't harm the metal.
You will still probably need to make a wooden wedge to tap between the barrel and cylinder to force them apart. That's can be as simple as a wooden clothes hanger whittled to fit. Hang in there, have faith in yourself, and you'll have it working again before you know it.