old colonel, I live geographically on a raft jutting into the North Atlantic, salt in all its forms everywhere. With a modicum of care---a pull and a swipe or two, oiled sparingly---none should worry about rust or sodden wood, muzzles up or down. Mine are muzzle-up, standing as for inspection, not as wounded on one leg. My early firearms look the same as when I started with them 75 years ago. I've never seen guns mounted like crutches in elegant private gunrooms.