I do think that shotguns have some utility as a cavalry weapon, despite the fact they seem to have been mostly ad-hoc weapons in the ACW. In fact I might even say that for close-range fighting I'd prefer one over a carbine. It's hard enough to hit a target while on a moving horse, and a shotgun increases those odds as long as you're within range. Even if you don't hit the rider, a blast of buckshot to the horse's face could cause trouble for your opponent.