Depends on what you mean by "rust" and what you want, afterwards.
Light spotting: 0000 Steel wool, and gun oil.
More: Try above, and see
Remember that blueing is rust. If it's so bad that you're worried about effacing the SN.....well....that's rusty!
If you provide more info about the gun, the extent/severity of the rust, and the eventual use of the gun, you'll get better replies. An H&R Top-break revolver in .32 Wimpy will require a different standard of care than a Parker G grade in an original leg-o-mutton case.