With a Flinter, nothing but real blackpowder should ever be used. The subs just dont get it done.

With your caplock, remember to hold it cocked to the side when pouring the powder and then smack it a couple times on the side of the barrel to help shift some powder into the drum "What the nipple screws into"

I only use the stainless steel Spitfire with Hollow base Nipples from Cabelas, I feel they offer better flame to powder.

2" groups @ 50 isnt bad, it will only get better with practice. Also remember to not over prime. Start off with as little prime that you can and slowly work your way up until you find the charge that goes boom instantly.