I have never found AP to be a consistent powder. I think if you used something like Olde Eynsford, Swiss or even Goex as your primer, you will diminish standard deviation and increase ignition reliability. Clean with Ballistol / water mix. I use OE as a booster at 15grs and 50 grs Triple 7 in Marlin 444 Black Powder cartridges.
The no 10/11 percussion cap is plenty for any 1858 load. A 209 or pistol primer in the 1858, if you could even work it out, it's way too powerful. It would knock the charge halfway down a 7.5" barrel before the powder would even start to ignite!