Sorry, I missed the USA part in the first post. I have been looking at the Wilson's Creek action for other reasons.

Cavalry was undergoing a major change in the ACW. Sabre tactics were obsolete as pistols were much more effective than sabres at very close range. Cavalry found it difficult to form properly for sabre charges (if the ground was open enough to do so) because rifles made them easy targets. In thicket country revolvers were often the most practical arm and superior to the sabre, carbine, or rifled musket.

Cavalry was transforming largely into mounted infantry. The carbines and shorter musket variants gave them the ability to take cover and hold positions dismounted (something they really couldn't do mounted.) As such they could advance to a position and hold it until infantry came up. The rifles improved the reach enough that the cav really didn't need to fear bayonet charges by infantry any longer. Breech loading carbines or repeaters even gave them a firepower advantage over infantry at range.

While the carbine could easily be reloaded while mounted, sustaining a firefight in the saddle seems a poor option other than for pursuit. If stationary and firing, the valuable horse and its rider presented easy targets.

It was very common for CSA cav to use Mississippi's, Lorenz, and other shorter-than-standard muzzle loader variants. It was also common for them to serve as dismounted infantry. How much this extended to U.S. cavalry in the trans-Mississippi I do not know.