[skeet]

I know this is a messy idea but take the grips off and anything plastic and immerse in kerosene for about 12 hours. It should free everything up . If necessary after the soaking push the cylinder catch and have someone tal the cylinder with a hardwood stick or a brass hammer. Once you get the cylinder open you will really have to take it out of the gun and clean everything possible. Even the kerosene may not stop the rust from the blackpowder. I used this method on some blackpowder handguns back when I was working on guns.