One little note: Horses are pretty hesitant about trampling people.
I'd like to suggest again, being prone should have SERIOUS disadvantages and leave you vulnerable for a sudden attack.
1. Extreme slow reload. At least double the normal time.
2. Standing up taking more then just a few seconds and leaving you totally vulnerable all the way.
3. Impossible to change to melee stand while being prone.
Being prone should give the player a little extra accuracy, but nothing more. Being a really small target is enough advantage.