It is my belief that what 7.62NATO wrote is 100% correct.

Last season, i hunted in rain without covering the muzzle of the rifle i was carrying. I usually cover the muzzle of rifles, when i am hunting, but for some inexplicable reason i didn't that trip. The rifle was carried in the rain on my shoulder with the barrel pointed to the sky. I didn't think about it afterward either. My failure at taking care of my rifle is inexcusable. Too much fun around the campfire perhaps. A month later, or more, the gun fired normally. The powder used was BH209. When i went to clean it i was shocked to see that where the powder was located, the barrel was quite rusted. When i finish typing, i will head out, and hunt with that very same rifle. It shoots good.

Whenever i hunt in rain or falling snow, i (always) cover the muzzle of my rifle.