I have hunted in pouring rain and freezing rain, and snow with a percussion and I can say they can be 100% water proof. One time in freezing rain, by the end of the day there was a sheet of ice covering my rifle, (a "sporterized" Zouave) and although I had kept the cap and hammer area clear of ice, it was totally soaked. Usually I don't fire off my rifle at the end of the day, but figured this would be a good time to do this. She went "bang" totally normal, but all the ice shattered off at the shot!! Wish I had had a video of that!
Anyhow, IMHO a tight fitting cap will keep yer powder dry. I've never used a kap-cover, but if they have to be taken off before you can fire....?? If so that would kind of rule them out for me.
I have carefully put a little bee's wax on a musket nipple, and then seated the cap, being careful that none got up into the cap or IN nipple. With a #10 or #11 type cap one could seat the cap, and then put some wax around where the skirt of the cap meets the nipple. Can't imagine how that would be anything less than 100% water proof. Should be able to lay the gun in the bathtub, take it out and fire it.
I think that many times when people believe the gun has "drawn" moisture into the main charge, what really happened is that oil was left in the breech, and water had nothing to do with it. A light rain especially should not cause any percussion rifle not to fire, or hang fire.
One thing I learned a long time ago, but people tell me the caps are different now, is that snapping a cap or two to "clear" the oil out, before loading for a hunt, caused a fouling that attracted moisture. In other words, IMHO, snapping caps before loading was causing miss-fires. Again, I'm told that the cap fouling on the current caps don't produce a hydroscopic fouling. But, there's still NO WAY I'd snap a cap befor loading up for a hunt. At the range yes. Hunting no.
Carefully dry all the oil (or soap suds) out of your breech and flash channels, and use a tight fitting cap, and don't snap any caps prior to loading for a hunt, (flame suit on) and I believe you can hunt in any weather with your percussion rifle.
Rat