I’ve been considering trying musket caps but with the comments above I think I’ll hold off on that. I use CCI magnum #11 caps and they work fine. Zero delay between little bang and big bang, just one bang. The few misfires that I have had were my fault, either:
1. The red chemical in the cap had fallen out and I didn’t notice
2. I didn’t clear the oil from the nipple and snail by snapping a couple caps prior to loading
3. I didn’t put any powder in

I use FFFg powder in my .54 – I know this isn’t recommended but I spoke with a Hodgdon technician and that’s what he uses. He of course gave the usual disclaimer to follow their recommended practices and if I blow myself up they have no liability, etc. The finer powder flows all the way to the base of the nipple for an instant light and I use less than a 100 grains, nowhere near the max for my rifle, so overcharge should not be a issue.
The only complaint I have about the #11 caps are that they are small and hard to handle.