I guess I would heat some water up to boiling, and pour it down the barrel with the gun pointed upwards. If the breech is sealed and the water doesn't run out, let it sit for 10 minutes and then pour out the water and try to pull the ramrod out with a vicegrip type of pliers.

Be careful when you handle the gun after the boiling water, it will be hot!

Other people may know of a better solvent, but you won't hurt anything trying that