In addition to the other recommendations offered, I might suggest trying Flitz metal polish (use it sparingly). Weapon Shield also seems to work well on rust, based on my experience.
Just hit it with a high quality cleaner/polish and a high quality brass/bronze brush and go from there.
Now, if you notice any pitting at all, I might seek some professional "intervention" at that point. But for light rust, a nice, thorough cleaning should do the trick.
Also, as has been mentioned before, perhaps listing the make/model (and if possible, the year of manufacture) of the gun may help in gathering more accurate and useful cleaning suggestions.