Well, it certainly looks, uh, salty.
I would disassemble as far as you are able, bead blast if you can, otherwise scrub the dog snot out of it with steel wool and oil.
Once you have it as clean as you can, either paint it (barbecue/stove paint is cheap and works pretty good), or better yet, use one of the various gun coatings available from Brownells.
None of this is any different than what the above gentlemen mentioned, but I will add that without aggressive treatment (bead blast for example), it will be pretty difficult to get a good finish again. Even after scrubbing and bead blasting, you may well see pitting.
If you neutralize the rust, you can always just let it go as is and keep it well lubed. Sometimes a "salty" gun is pretty neat looking.
Tips for next time. Don't store in your trunk. If you must, check every day or so, rust starts FAST. Last, consider giving the whole thing a real good coat of paste wax to help avoid such accidents in the future.