The owner's manual says THAT rifle is water soluble, and you should never get it wet.
OK- just kidding. Assuming that you have a blued firearm- bluing IS a form of CONTROLLED rusting. (black iron oxide instead of red). I have firearms that are over 100 years old, and still in very good condition, despite having been wet a time or two or three.
The keys to a long finish life are (1) apply a LIGHT coat of a decent rust preventative BEFORE you take a gun to the field, (2) when returning with a wet gun, dry it promptly. Does not have to be a full breakdown and clean. Dry towel, followed by a wipe with an oiled cloth. But a rainy day followed by proper care should not do significant harm to your gun.
Myself- a wipedown with CLP or Break-Free. There are some products (Frog Lube being one) that will help shed water.
I'd like for you to think for minute about military forces, in the field, during monsoon season. Their stuff gets WET.