Finding rust on your firearm is always a bad sign. Poor storage or neglect are often the cause, but here’s one solution old timers have been using for years.
In the video below, the shooter shows off a double barrel he picked up at a Las Vegas gun show for a great price because the outside of the barrel is a little worse for wear.
Using a secret from an old gunsmithing book he shows a way to take off the rust without hurting the bluing using nothing more than some light oil and a copper penny (pre 1982).