Goex FFFFg will work great in the flash pan as your priming powder. Of all the black powders out there, Goex, Swiss, and others, I use Goex. If your rifle seems like it is having a slow ignition with the Triple Se7en as the main charge, I was told to drop a little (like 5 grains) of the Goex in first to help the Triple Se7en ignite.

I am thinking of buying another rifle today. I guess all the other rifles need company... It is a .50 caliber flinter. It sure looks like a nice rifle....

Good luck shooting yours...