You might consider wiping Rain-X on the metal of the gun. Allow it to react for a few minutes and wipe it back off. That material will form a chemical bond with the metal and will help it to repel water and resist moisture pick-up. 3M used to make and sell an aerosol version of a similar water-proofing called ScotchGard. NASA used such a waterproofing to treat the ceramic tiles on the Space Shuttles.