If anything, I have found the CCI #11 Magnum caps to beMORE POWERFUL than musket caps- at any rate, they are at least equal. There is one big advantage for the musket cap - it is easier to handle in cold weather. This is negated if you use a capper, though....

I have one rifle that has a custom-made lock, and the guy who made it cut the half-cock notch so that the hammer stands close enough to the top of the nipple to prevent a cap from coming off, yet far enough that it does not touch the seated cap - safe, but it secures the cap in place (the maker was a pretty bright fellow....)