Put the breech end of barrel in an ultrasonic cleaner and run it a few cycles, if you have access to one, I'f that won't free it up most likely it won't ever come out.
Will it fire? If it won't, you may be able to push a piece of wire through it to allow it to fire, and then fire it 3-4 times to heat it up. It may break free afterwards.
Penetrating oil is also your friend as some have said.