I've spent a fair amount of time hunting in the rain because I wanted to learn what the problems were. I picked out rainy days to squirrel hunt, and it was a real learning experience. It was worth the effort, I've taken quite a few squirrels, one turkey and a couple of deer while hunting in the rain.
It's no big job to keep the gun dry if I load it in the dry and then end up hunting in the rain. A cow's knee is what works best for me, but just sticking the lock under a coat, in their armpit, or anything else works for different guys, and even in some pretty heavy rain.
It's after I fire and need to reload in the rain that I find the biggest problems. If it's really raining, not just sprinkling, it is frequently impossible. I was surprised to find my powderhorn spout wet the powder as it came out, both my main horn and my priming horn. If the spout stopper isn't waterproof, powder near it inside the horn will get wet and clog so none can come out. Water will go down the bore as well as into the pan. The pan will be wet enough to destroy the prime when I drop it in. The only way to avoid all these problems is to get under some kind of cover, either natural or my own, hunching over, shielding with a broad-brimmed hat, spreading my surtout on a bush, &c. And I have to keep something dry with which to wipe the pan and the powderhorn spout, or forget it. Even if I manage all that, it's sometimes surprisingly tough to take an accurate sight with heavy rain bouncing off the barrel.
Even the old boys who's very life depended on it, could not always avoid the problems. Daniel Trabue, Logan's Fort, Ky., 1780, bear hunting for food:
"It was raining. I was afraid to git my gun wet. So I road close to him and stooped over the gun and fired at him not taking sight. Away went the bear. The dogs soon stopped him. I went off to a large tree and loaded, and the next time I thought I would go very close, and road up close and my gun flashed. The bear Jumped at me, scared the horse so he Jumed side way against a Dog wood. I took up the dog wood. Dropt my gun. Away went the horse and bear. I picked up my gun, ketched my horse, and went to the bear. My gun was wet. I could not get it off. I made many trials but all in vain."
Spencer Records, on the Licking River, Ky., 1784:
"We there saw the fresh track of an Indian, which we followed for several miles. That evening a heavy shower of rain fell, so that both our guns got wet, which rendered them useless... As our guns were wet and out of order, we let them remain so; which I think was providentially ordered; for if we had put them in order that night, which could only be done, by picking powder in at the touchhole, and shooting them off; the Indians would have heard them, and have come in search of us, and found us by our fire."
Spence