Keping it dry is th esecret. There are small rubber or plastic cap covers you can buy to skip down over the nipple and cap, or use some wax or duct seal,which also works.Tape the end of you barrel with electricians tape ,just as you would a center fire rifle. Seal that barrel as well as you can.
If the humidity is high or raining.I pull the load every evening (I don't shoot it) and run a pacth or two with methanol down the barrel to absorb any moisture.Then I give it a light coating of Bore butter, swab withadry patch and reload.
I have always used standard #11 percussion caps and don't remember any ignition problems. I stopped using Pydodex years ago when 777 came out,but I have gone back to black powder.My inline,I use 209 primers.
