One trainer (name lost) wrote in an article that there are Four Fears in CMS:
1. Balloons
2. The sound of the "clock mechanism" of the pistol.
3. The report of the shot.
4. The cloud of smoke.
In addition to the comments made on noise, the trainer must accustom the horse to rding the pattern with balloon flopping about in the breeze on the end of sticks. This trainer also recommended that when training begins do the first firing without the balloons. Just ride random patterns and fire at random times. This will prevent the horse from associating the presence of the balloon with the upcoming shot. That should prevent the horse from shying at the sight of the balloon (a real hassle if it starts to happen).
Lots of "dry firing" is a Good Thing for both horse and rider. It will allow the rider to work on an effective technique for handling the pistol, picking up the target, timing the shot, etc. The horse will generaly settle and get used to the noise without associating it with the sound of a shot.
The cloud is sometimes the toughest thing to get the horse through. Most trainers seem to recommend you fire to the off side, at 90 degrees or so to begin with (so that the cloud is well behind the horse). As the horse gains experience you can begin to move forward (maybe up to 60 degrees or so). And when the horse is comfortable with (or at least accepting of) this they you can add on side targets. Note that the above presumes a right handed shooter; south paws have to do it in mirror image.
Some trainers say it take 1000 rounds to make a shooting horse. They just might be right!!!!
SQQ