As much as I accept that the Napoleonic-era rifle/musket/whatever was grossly inaccurate, when I read accounts like here, I become skeptical. When I read lines like:
"Napier claimed that in Spain he witnessed volleys
fired by British infantry where out of 300 musketballs
fired none hit the target."
I become EXTREMELY skeptical.
Unfortunately for me, the only thing I have to go on is, essentially, casualties.
The Battle of Waterloo, with these grossly inaccurate muskets/rifles, had about 45,000 casualties with one side numbering 72,000, and the other side numbering 68,000, then an additional 50,000
In comparison, the Battle of Gettysburg, with insanely more accurate rifles/muskets, had about 50,000 casualties, with one side numbering 90,000, and the other side numbering 70,000 (at best)
Considering the immense differences in gun technology, I have to roughly assume that Napoleonic-era guns are simply nowhere near as inaccurate as the impression I'm getting in my head. Though being autistic-y, I should say the impression in my head is essentially that of Empire Realism---where a single volley at maximum range kills nothing, and another volley at middle-range kills about 5-15, out of a unit of 200.
So where do these massive casualty figures come from? I sincerely doubt it could be from melee combat, or artillery fire, or cavalry charges, meanwhile Gettysburg was almost entirely an infantry affair for the first two days.