Also, after you have cleaned the devil out of it.. be sure to put a good heavy coat of oil in the barrel and let it sit and soak in. When you really clean a barrel good, you have basically taken them back to bare metal and so you need to protect that metal.

I personally like to pour boiling water through the barrel to heat it up real good, then after making sure the barrel is dry with some patches, put a saturated oil patch in there and swab that bore. As the metal cools, it will draw in that oil into all the little pours and protect it from further rusting.