Mountain Devil has a great write up on the o-ring:
https://hunting-washington.com/smf/index.php/topic,239607.0.html Thanks Mountain Devil for your inputs and assistance on muzzleloaders.
I hunt wet side, and even with BH 209 I've had water in the barrel in the heavy rains. But I'm going to try out some of Mountain Devil's ideas as well as better handling ideas from Damnimissed.
Some lessons from my brief muzzleloading years:
Electrical tape over the barrel can cause condensation with temperature variations and high humidity. I've had rust build up at that tip of my barrel on one of my modern rifles because I left the tape on too long years ago. Lesson learned. I change the tape at least once per day and make sure I keep it off while in the tent so it can dry a bit.
I take out the powder ever night, let the barrel dry out, and put in new powder every morning. I keep my BH209 tubes in vacuum sealed bags in the tent, except for the two spare tubes I put in my front pocket in the morning.
Don't use the pellets. My buddy did, and had a 5x5 bull at 25 yards broadside and "pop" cap went off but not pellets. Unfortunately I wasn't in position to get a shot off on the running bull with his cows around him. Still a great day to have an elk herd walk in around us so close. And to be fair, I don't know if my BH209 would have gone off in that nasty heavy rain storm either as I didn't shoot it off, I pushed the powder out (as we camp in the backcountry).
We've researched some of the old time leather covers over the breech that some of the flint lock guys use to put over the breech, but haven't found one commercially available. Most guys make their own. Am still thinking of trying something out there.
But there are far far more experienced blackpowder hunters on here than I so I'll let them chime in.