.28 is about .54. I have a .54 cal. bl. for my pistol, that does amazingly well with .350", what Hornady calls 000.
Triple Ought is actually .36" but not from Hornady.
I used a patched .526" round ball, dropped 3 buck on top, then showed a patch on top of that to hold the balls inside.
I shot several 'patterns' at a B27 Silhouette target at 12 yards. All of the balls landed insid ether 8 ring, every time, with the full sized ball
striking centre 10 ring and the buck shot striking radially in the 9 and 8 rings in about perfect symmetry around the centre ball's hole.
Your army guys of the 1700's and 1800's used double the number of buck and ball compared to single ball loads in the muskets - are you allowed buck
and ball in this match or are you restricted to 3 small shot, buck size or smaller?
I would think that 0 buck, at .30 cal. might be about the correct size. #3 buck is around .28" isn't it? That might be better yet- however they would make a VERY light load,
projectile, wise.