Okay, back to the original question. I have used a flintlock for hunting and target shooting in the rain and on very damp days. By rain, I mean at worst a light drizzle. I won’t hunt or target shoot in heavier rain. Since my target shooting is done from under a shed, that wouldn’t be a fair discussion.
But when squirrel hunting, I usually have on a heavy coat and a wide brimmed hat and I just tuck the gun up under my arm or one side of the coat to keep water away from the lock. After firing, I wipe the pan, flint and frizzen with a dry rag which is kept in my pocket. This is a very important step. I still may get an occasional klatch because of carelessness and not keeping moisture away from the lock and when this happens I just dump the prime, dry off the lock and start again. Most squirrels don’t mind if I don’t make a big ruckus doing it.
Also, I use 2f or 3f for the main charge and to prime. Just have to use one horn that way.
I’ve never used a cows knee so can’t help you there.
So yes, flintlocks can be reliable when it’s raining. And if you learn to shoot them in the rain, you can shoot them anytime.
Hope this helps.