There are chemical derusting agents available that do a pretty fair job of removing rust but they are acids and you might get a mild etch. X-Rust-O is the name of one I believe. Actually though I'd try soaking them in white vinegar. It wont really cause any more damage than has already been done by the rust. Just make sure to thoroughly wash them in hot soapy water to remove any residue when done, dry, then use a light coat of oil to prevent further rust. Clean the oil out of the bore before shooting them of course.