Okay, I wasn't sure if you were saying Track was out of flints...which would surprise me, but with all supply lines in chaos I wouldn't be surprised. !!
Six to eight at 50 isn't bad for the first shots tried. You'll get down to 3" at fifty in no time.
My Brown Bess is most accurate with a heavy charge, rather than lighter. Also, it seems to prefer a small ball, thick patch combo over a thin patch, large ball. Even though it's a .75", I shoot a "chewed" .690" in it. I've tried bigger, but the .69 shoots best. "unchewed" I use the denim patch, "chewed" I use a .023" tick patch. Also I put one pure wax, waxed felt wad over the powder, and a slightly lubed wool wad over that. But them smoothies is all different. !
How did the .575's go down with ticking? Have an idea what the actual bore-size is?