I agree. I have a hard time believing that a black powder rifle can reach that far, even under perfect conditions.
With muzzle velocity in the neighborhood of, what... 1500 fps or so? At 200 yards it’s likely shed about half of it’s energy and it’s going down quick. Fired at a 45 degree angle, I doubt it would reach beyond 800 yards, and at any rate at that point it is merely falling with gravity with little if any momentum left. A falling bullet is orders of magnitude less lethal.
I suspect that there’s something not reported here, or misreported. In any case, I suppose, it’s a bizarre and wildly unlikely circumstance and very sad indeed for the girl.
"....use the 250-grain Partition-HG in an inline muzzleloader capable of handling three 50-grain Pyrodex Pellets. Some time back I tried that combination, and it averaged just over 2100 fps for 2475 ft-lbs of energy at the muzzle".
A pointed nose 250 gr bullet (enclosed in a sabot) at 2100 fps vs a round nosed 30/30 170 gr bullet at 2200 fps, my guess is the maximum range would be about the same. I looked at a box of 30/30 rounds, but a maximum range warning was not printed on the box.
A few years ago I did see on a box of 140 gr 7mm Remington Magnum rounds a warning of a maximum range of 3 1/2 miles.
50 years ago the 22 Long Rifle rimfire warned of a maximum range of 1 mile. Today 22 Long Rifle boxes say: Dangerous to 1 1/2 miles.
Perhaps someone knows or has seen accurate information as to the maximum range of a modern muzzle loader, I have not.
You need to study your ballistic tables. A heavy slug loses velocity more slowly than light ones. Yes, a buffalo bullet will be lethal at over one mile. The trajectory will look like a howitzer but it will be lethal.