Your lucky Wyoming is pretty dry so its not as much of a problem as more humid parts of the world. The acids in the leather reacts to humidity and you cant do much to stop it veg tan has tannic acid in it. I don't store firearms or ammo like that anymore but you sure can if you want. You will have to check it once in awhile and replace or clean the rounds, really the only way to prevent the stuff is good periodic maintenance and that is a necessity with the firearm as well. Moths and other bugs are notorious for getting inside a gun barrel and dyeing, when they do they can ruin the rifling, throat or cylinder for that matter in short order. so if your holster is open toe its something to think about also. I don't polish my brass before reloading so it always has a patina lol. Good luck friend.