Just use common sense. I wouldn't intentionally take my best double out in a heavy rain, but I wouldn't quit hunting if a surprise storm came up just because I was carrying a nice gun. I do own some Rainy-Day-Loan-It-To-Your-Brother-in-Law guns. Some of my best hunting days have been during days of steady rain. I've spent many days in the rain deer hunting with a flintlock. It's cool when I have instaneous ignition on a rainy day. But there have been times when I found black mud in the pan at the end of the day and wondered how many hours I had been carrying a dead rifle. I do what I can to keep myself and the guns' innards dry, and take care of business when I get home. Normal cleaning with the flintlock is to disassemble and clean barrels, lock, etc. with hot water, dry thoroughly and oil. Water never hurt these parts and water won't hurt shotgun parts. But neglect will.
I would not fire an unfired high grade collectible double just to do it. Nor would I lift barbed wire with the barrels of most any gun. I have endured cuts and scratches on my hands and face while busting through briars and multiflora rose in order to protect my gun's stock because I figure those little scratches on my hands will heal in short order while they would scar my gun forever.
But in the end, you could have your high grade gun out on the brightest blue-bird type day and slip or trip and bust your stock through the grip or ding your barrels. I think those things probably ruin more guns than hunting in the rain.
