Although I use FFFg for both the main & priming charges in my .50 rocklock, most use FFg / Holy Black for the main charge and either FFFg or FFFFg for the pan.
Pyro "RS" (rifle) is the BP substitute for FFg, but flinters don't do well with subs, as a general rule.
Pyro "P" (pistol) equates to FFFg - I would save it for a cap 'n ball revolver.
FWIW, BP subs are harder to ignite than Holy Black, usually moreso for the spark from a rock than a cap/primer.
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