Before a shootout, Butch reminds Sundance that their Colt revolvers are each loaded with six rounds. Although such pistols could indeed hold six cartridges, common practice at the time was to load only five, thereby keeping the hammer down on an empty chamber. This was a safety measure intended to prevent the gun from firing if it was dropped on the the hammer with six rounds loaded.
The initial scene shows a fast-moving posse mounted on horseback chasing Butch and firing at him. But the constant erratic motion of their horses would have prevented the riders from aiming effectively. As a result, only a very lucky shot could have hit Butch.