I'm usually fighting on horseback, and using either a repeater or a shotgun (plus a sabre), so if you do so too...here's my priority list for enemies to shoot.
1.) Anyone aiming at me
2.) Anyone aiming at my men
3.) Anyone travelling straight towards or away form me
4.) Anyone in optimal firing range (close enough to not need to lead, far enough to not have to follow him to quickly)
5.) Anyone else
While this won't help with hitting things, it will instead make you get hit less often. And ensure that as many of your men as possible live to do the shooting instead

PS: Always charge your horse into ground units trying to reload. Ha ha HAAA CHOP SLICE SPLASH ouch!
PPS: I'm slow today. Easy solution: Use
shotguns. Double Eight Gauge if you fight on foot (one shot kill machine!), or any of the others if you're riding. You don't really have to aim as well with them, just go close and stick them as tightly into your enemies as possible.