First off, if loaded unload the one or both ....
I may be a bit over-kill but if it were mine and sat that long, I'd pull the barrel, mount and anchor it muzzle up. Plug the nipple or nipple plug end. Fill the bore with a liquid wrench just enough to cover the lower breech end. Plug the bore with a rubber stopper. Let it sit for about three or four days. Oh yes, make sure the breech end is sitting in a pan container. Monitor any leakage during the soak. ...
Clean up and try it again. If this doesn't get it, then the next step will require some heat and firing a few rounds. ...
There was a previous post about a home-made mix called shop-mix or something like that. It's 50/50 mix of acetone and auto transmission fluid. Sounded good to me but have never tried it. ...
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