Removing surface rust from a gun without damaging the finish can be tricky if you don't know how, but there's really nothing to it. Keeping your gun free of rust helps preserve your gun's beauty and value.
• Find some light oil, fine steel wool, and a suitable work surface that won't scratch up your gun.
• Check if the gun is loaded; if so, unload it.
• Apply some oil on and around any rust spots.
• Making sure to keep oil present at all times (add some if you need to), gently rub rusty area with steel wool.
• Wipe with an old rag or paper towel occasionally to remove rusty oil and to inspect the surface.
• Repeat as necessary until no rust is left.
• Apply a light, even coat of oil to all steel surfaces.
Tips:
• Always keep guns pointed in a safe direction.
• Never use abrasives, like sandpaper or emery cloth, on your gun.
• After removing rust, keep that spot oiled and keep your eye on it. It will often be the first spot to rust in the future.
When iron starts to corrode, it forms rust, a substance that often causes unsightly brownish stains. If you accidentally brush up against a rusty surface, you'll be rewarded with a serious laundry stain. Follow these stain removal tips to give your fabrics and household surfaces a clean start.
Sources:
http://hunting.about.com/c/ht/00/07/How_Remove_Rust_Gun0962933349.htm
http://en.wikipedia.org/wiki/Rust
http://home.howstuffworks.com/how-to-remove-iron-and-rust-stains.htm