those on a real .50 caliber machine gun. When the image is thrown on the screen, they can see the wing tips of their own bomber, as well as the approaching fighter. They lead it in their standard Mark 9 reflector sights, pulling the trigger when they think they are on the correct "point of aim." If they are in a free-gun position, instead of a turret position, the gun handles start jumping, the vibrations carefully reproduced to simulate those of a real gun. Through earphones they can hear the authentic sound of their own guns. When a hit is scored, it sounds a 1,000-cycle "beep" in their ears and moves a register on the instructor's control panel. The Army Air Forces also use the Waller Trainer.