Get some 0000 grade (ultra fine) steel wool at a hardware or paint store. Don't use any other grade or anything from under the sink. Make sure the gun is unloaded. Put some gun oil or penetrating oil on the rust and scrub lightly with a small (1") ball of steel wool. The wool will break down as you scrub, turn it and replace it frequently. This will not remove the blued finish or scratch the steel. It will remove all the rust and expose any pits, possibly making the gun look worse. If the rust is very light it may be removed completely and will look perfect. Be careful not to get any wool residue into the inner workings of the gun unless you want to disassemble and clean it out. You can also wrap a bronze barrel brush with steel wool to remove light rust from the bore. Don't fire a gun with a badly pitted bore until it has been checked out by a gunsmith. All blued steel guns must be kept oiled to prevent rust.