Listen to what driftrider said. He gave pretty good advise. If the chamber has heavy rust, you may not be able to save it. A light coat may be able to be cleaned up, but in any case, do a reasonable cleaning, and please take it to a good gunsmith for a professionable opinion. Not only is the case body area involved, but the neck area too, and most likely the bore. You need to have the bolt examined as well.