BP subs rarely work reliably in flintlocks...I used Pyrodex for years with percussions and had to switch to Goex when I moved to flintlocks...and I found Goex to be so fast, clean, and accurate that I've rezeroed all my percussions to Goex as well.
You can use BP subs in a "duplex load" configuration...ie: 10-20grns Goex first (to get flintlock ignition) then some amount of 777 poured down on top of it...I used 20 + 50 at the range to use up some leftover Pyrodex, and they were accurate, consistent, etc.