My question is - would artillery actually waste shots trying to hit riders a mile away? Or was this just dramatic directorial license.
As far as the movie is concerned, I have no idea it could have been meant to show that, or it could have been edited together that way,, who knows. But I think all things considered the Union artillery had better, more pressing things to aim at.
But as far as an artillery piece being directed to fire at enemy officers, that is known to have happened. A perfect example is Leonidas Polk's death during the Atlanta Campaign. During a quiet period Polk, General Joseph Johnston, and General Hardee were checking the Confederate defenses on a small rise called Pine Mountain, and almost directly across from them a little under a mile, (I think, not sure if I'm remembering the distance right), General Sherman was likewise checking on one of his artillery positions, and
supposedly, spotted the three Confederate generals through his glasses, (got to poke fun at
Gettysburg's Pickett), and declared "How saucy they are!" and immediately ordered his gunners to train their guns on them, with the second or third shot ripping Polk apart.
If true that sort of thing happened but the circumstances are wildly different, mainly the part where the Union artillery wasn't facing a large Confederate infantry assault when they fired at the Confederate Generals a ways off on Pine Mountain.