Are you sure it was the rain? Blood, apple peel, salt from potato chips etc could have been the problem. You may also want to check the gun case. If the gun case was exposed to salt water on an earlier trip, that could be the problem.
I have used a little bit of Brasso mixed with Rem oil to remove rust and it did not harm the metal surface. I have tried this on bluing and case colors. Start out with a very small amount of Brasso compared to the oil. About 10 to 1 is a good starting point. Good luck.
Mark
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