I use Sellior and Belliot No11 caps on my Uberti 1858 and they fit like a glove. I have never had a misfire with these caps (touches nearest wood!)

I also use Triple 7 and don't use any grease over the end of the cylinder, just a pre lubricated wad. I find that this keeps things much cleaner and is easier to load. As long as your balls fit good and your nipples are pert then I wouldn't worry about flashovers