Kerosene and oil do a pretty good job of helping to weaken the rust. Use some of the rust dissolving oils like Liquid Wrench, Break free, Kroil, or a new product call Evaporust to dissolve the rust. It really depends on how much rust is in the barrel as to how well any of these products will help. None is going to give you a NEW barrel back again. If the rust is light, you should be able to remove it with some of these products, and a bit of elbow grease, with some lapping compound. ( auto parts store sell this.)
Then, shoot the barrel. If it shoots okay, you are in business. Just remember to clean it after every session, and then use a good oil( non-petroleum based oil) or a long term storage product like Birchwood Casey Barricade(formerly called Sheath) to protect the bore. Store the gun in a cool, DRY location, and check it once a month to be sure you didn't miss something. :thumbsup: