Depending on the kind of rustit is. If it is very light rust,I like to use (blue wonder's) gun cleaner. It removes rust without removing the bluing (itwon't evenremove new bluing)

what you have to do is get the finest steel wool that you can find like 000. then you get out your (blue wonder gun cleaner) generously apply it over the area where you see the rust. Then take your fine steel wool and rub lightly over the rust (working in your blue wonder gun cleaner)

Then, get out a blow dryer and heat the area up that has the gun cleaner on it (this will open the pores in the steel and causethe gun cleaner toreact)....after this take a very clean cloth and wipe off all the gun cleaner. (Your rag should have a rusty brown color on it.) Then apply one more coat of gun cleaner on the area and heat again. You dont have to use the steel wool again, unless the rust is bad. After wiping the second application of gun cleaner off, oil your gun like you normally would when you put it away.

Some guys also take a good oil and rub steel wool over it. This also works if the rust is not severe.

If the rust is severe...you can try to cold blue it yourself (which is not the easiest to make blend with the surrounding blue) because cold blue tends to be blotchy.
or you can take it to be reblued professionally...(this would suck)[:'(]...

good luck ...I really hope you can remove the rust without much hassle...I would hate to see you have to go to other means.