Outside of a few select units like the USSS and the later CSA sharpshooter battalions the soldiers did not get any proper training.
In the units that did get proper training it was possible to engage artillery at 500+ yards. Their hit % would be very low, but even loosing a few men, and just as problematic their horses might drive off a battery...
But at that range the built in "spread" of gun, powder and bullet make sure that no matter how good a marksman you are, hitting is also a question of luck.
(just like hitting at 400 yards with a M4 and standard 556 ammo involves luck)
When trying to hit anything at 300 yards or more you need to estimate the range with less than a 50 yard error or you will miss. This is not easy and it give you a much smaller margin compared to modern firearms.
btw the Pritchett ball was only used for a few years in British service. it was extremely accurate in European climate under perfect conditions, but it was horrible for military use in the rest of the world where it fouled the gun after only a few shots.
So the British changed to a smaller bullet, that worked almost as well under perfect conditions... but much, much better under battlefield conditions in, say India.,
This is a very good article about it:
https://nebula.wsimg.com/a1ee6168412...r6_FopBrwlKo-g
It also cover the fact that the rifle musket was not a new weapon by 1860 and that Minié do in no way earn the fame he got...