I use 3f Goex black in my 50 cal flinters. It burns cleaner than 2f. I use round balls. 3f load is ~ 70 grain. This is a 15/16" x flats .50 cal Green Mountain x 36" long barrel. 3f generate higher pressure than 2f black. I wouldn't go over 80 gr of 3f in my 50 cal x 15/16" diam. You could probably go to 90gr+ with 2f-but since I don't shoot it, I couldn't tell you for sure-check you loading books (ie Lyman).

I used to shoot Pyrodex in my 50 cal caplock. First I tried the RS grade ("2f"). It had severe delay time. I tried Pistol "P" grade ("3f") the next can, and the delay way just about eliminated. Haven't tried the 777. I went to Goex blackpowder after that, and have lightning fast ignitions-even with my flinter.

For me, it's a cleanliness issue. SIGNIFICANTLY less fouling with 3f black vs 2f black.