A starting pistol or starter pistol is a handgun (typically a specially designed revolver) that is fired to start track and field races, as well as competitive swimming races at some meets, and the sound of the gun going off serves as the signal for the athletes to begin the event. Blank shells or caps are used to prevent injury, and thus usually a cloud of smoke can be seen when shot. Starting pistols may be modified versions of standard pistols incapable of firing bullets, most commonly achieved by welding an obstruction into the barrel. When electronic timing is used, a sensor is often affixed to the gun, which sends an electronic signal to the timing system upon firing. For deaf competitors or for modern electronic systems, a light may be used instead.