I carry a spray can of the stuff in my shooting bag. I never plan to shoot a gun in the rain but I have been caught by surprise a few times at a shoot or a match and I spray the metal surfaces of a pistol or shotgun with it, wipe off any drips, and go. It hasn't bothered any wood or plastic that it has come in contact with so far.

My favorite moisture displacing spray used to be Corrosion X. I still use it for electronic connections on the boat. I saw a demonstration at a trade show where a solid state TV chassis was sprayed with the stuff and then submerged in an aquarium where it worked underwater. The bad part of spraying a gun with it was that it made the shotgun stock or pistol grips slippery unless you wiped it well. It also never damaged wood or plastic.