There are badly trained horses out there, and firing guns on most that aren’t trained with them will cause them to flip out and knock you off.
I fell off a horse once and was blacked out for a little while from the impact. So guns on untrained horses is a no-go unless silenced.
On the same horse without a saddle or reigns I was able to get the horse to walk around with relative ease, even with little to no training. But controlling one without gear is horrendous, this way and that. Always running amuck.