Mark, I've been a cap shooter and hunter for many many years. You do not need to prime the drum. All of the horror stories of hunters having problems with caps going off and not the charge, is from a lack of care, period. Follow these simple steps, and you'll never have a problem. I do all this the night before I hunt.
First, swab your bore, drum and nipple using patches with rubbing alcohol. I use pipe cleaners to clean the nipple and drum. This will remove the oils, which is the biggest culprit of misfires. Let these sit and dry a few minutes.
Second, swab again with dry patches and pipe cleaners.
Third, install the nipple, and head outside and snap 2 or 3 caps to make sure everything is dry.
Forth, wake up, head out, load your rifle, shoot deer, and enjoy the venison.
Seriously, these few easy simple steps will ensure your gun will fire in the morning. I use CCI #11 Magnum primers. I've never had a misfire on my cap gun hunting. A little prep before you go is all that is needed.