I just got a laugh at seeing my girlfriends face when I yelled boom boom boom. Haha
Minimon, what worked was that you gave them a distraction. Horses usually settle fairly quickly with repetitive sounds, that of the gun. To help prevent spooking, don't allow the horse time to think about spooking but keep his mind busy on what you are asking so it's a constant serpentines, circles, backing, even partially doing these so you are constantly bending the horse and moving the hind quarters.