I have a .40 Lancaster flintlock somebody made from parts from Dixie Gun Works that I got at a pawn shop. It has a 44" x 7/8" Douglas barrel, 1-66 twist. 40 grains of Goex 3F and four different sizes of balls did not group especially well. So I upped the charge to 80 grains of Goex 2F, just to see. Nine shots averaged 2191 fps (dropping the first of the 10 because it was about 90 fps slower), and at 50 yards from rest, 10 shots made a 2.6" group. Six of them went into 0.6".
A week later, same load of .395 Hornady and pillow ticking and 80 gr. Goex 2F with no fouler, 6 shots averaged 2178 fps and went into 1.75".
Next group of six the same except for "super muslin" patches, all holed or torn. Mean velocity was 2153 fps, group was 1.6".
Next group was .395 Hornady, back to thick pillow ticking, and 90 grains of Goex 2F. Five shots averaged 2303 fps, 42 spread, five shots in 1.5" with one blown patch, four shots in 1 1/16". Thus the Goex 2F gave me the best accuracy so far. I think this slow twist needs high velocity and while 3F might do as well, 2F did very well.
I have also used 40 grains of Swiss 2F in .40 calibers with good accuracy.