Probably not applicable to the rifle shooting you guys are talking about, and nothing to do with ballistic performance in the rain, but my experience with the mechanics of operating a firearm in the rain: 1977, was out in the hills shooting .22's with my buddy. It started to rain hard, but we kept on shooting at targets we had against the hillside. I was shooting a Ruger Mark I target pistol, which I had in a 2 hand grip. After the next round I felt a sharp pain in the bottom edge of my left hand, under the magazine.
Water had hit the surface of the brass of a round in the fraction of a second it was open to the air while chambering, and coated it as it entered the bore. The extra pressure blew off the cartridge rim, and drove a piece of brass back and down the mag well around the edge of the magazine and into my hand, leaving just a tube of brass in the bore.
Just FYI; doubt that anything would happen with a modern bolt action and non rimfire ammo, but this tells me that in the right conditions water on the surface of the cartridge can increase pressures.
This would probably be much more likely in an autoloader with looser tolerances (and maybe impossible in a bolt action, don't know), but still...