Ed Driscoll at InstaPundit has published a couple of photos of a mounted shooting competition. This is a fun sport, although you should take it with a grain of salt. The .45s in this competition are loaded with wax for safety reasons, and it disintegrates under the pressure of the explosive gas created by the cartridge going off. That's why, in the second shot, you see what appear to be rays of light going off in many directions: they are bits of burning wax flying through the air. As you can see, they don't go all that far before burning up and falling out of the air. That means it's safe to shoot guns in the arena with spectators all around you -- as long as they are more than a few feet away, there's no danger to them.
However, it also means that the revolver is acting more like a shotgun than like a revolver being fed traditional solid bullets. The accuracy of the riders is thus somewhat difficult to judge. Would they have hit the balloon if they'd been firing lead flat nose rounds? Maybe! It's still a lot of fun.