Corrected entry: Abbey's husband gives her a gun, but she only has three bullets. She puts them in the cylinder of her revolver one after the other. Later, when the PI steals the gun, he checks the bullets and sets the cylinder so that the first round is under the hammer. When he shoots Marty, the second round is fired because the pistol is double action; when you pull the trigger the cylinder is rotated. The PI then prepares to shoot Marty the second time, pulling the trigger, which rotates the cylinder and cocks the hammer, but doesn't feel the need to fire. Later that round is fired accidentally, and the trigger is pulled twice more rotating the cylinder twice more, with the hammer falling on empty cylinders. This should leave the next trigger pull on an empty cylinder, then the next should fire. It doesn't happen that way, and I think there is a trigger pull missing in the film.