I've never shot mounted CAS, but did a lot of hunting and just plain old can shooting along the roadways off of my Arabian. With him, our range was in his pasture, so he got used to it and often came over to see what we were doing. He kind of trained himself that way. Once I started working with shooting off of him, I used a cheap capgun, then went to my cap and ball gun with just caps. Figured if I had to drop a gun, they were at least cheap and probably unharmed in the process.
I finally ended up using my single shot .410, and after that, even a full house cap and ball .44 never phased him.
And I learned after one time to never ever ever shoot over his head. Bout got dumped over that one. DM