I put 200+ rounds through my duty G17 (Gen 3) while qualifying in the pouring rain this evening. Water was running through my holster while the pistol was in it......not a single hiccup, as usual......love it!
+1 for what "glide" said. Eezox or CorrosionX is excellent for what you're needing to do. The latter product is also one of the best lubes on the market.
Rain is also hard on our leather dog equipment, i.e. leashes, leather hidden sleeves, muzzles, etc. They all have to be dried and coated with Lexol.
I put 100rds. through my AK47 yesterday, also as it was raining. The difference here was that I was sitting in an easy chair on my back porch well out of the rain. The railing makes a great gun rest and the picnic table the perfect place for one's Jim Beam to set.