pendulum arms of exactly 66.25 inches (168.3 cm) in length, from bearing surface to bearing surface, and used turnbuckles located in the middle of the arms to provide a means of setting the arm length precisely. Ackley recommends masses for the body of the pendulum for various calibers as well; 50 pounds (22.7 kg) for rimfire up through the .22 Hornet, 90 pounds (40.9 kg) for .222 Remington through .35 Whelen, and 150 pounds (68.2 kg) for magnum rifle calibers. The pendulum is made of heavy metal pipe, welded shut at one end, and packed with paper and sand to stop the bullet. The open end of the pendulum was covered in a sheet of rubber, to allow the bullet to enter and prevent material from leaking out. To use the pendulum, it is set up with a device to measure the horizontal distance of the pendulum swing, such as a light rod that would be pushed backwards by the rear of the pendulum as it moved. The shooter is seated at least 15 feet (5 m) back from the pendulum (reducing the effects of muzzle blast on the pendulum) and a bullet is fired into the pendulum. To calculate the velocity of the bullet given the horizontal swing, the following formula is used: where: is the velocity of the bullet, in feet per second is the mass of the pendulum, in grains is the mass of the bullet, in grains is the horizontal travel of the pendulum, in inches For more accurate calculations, a number of changes are made, both to the construction and the use of the pendulum. The construction changes involve the addition of a small box on top of the pendulum. Before weighing the pendulum, the box is filled with a number of bullets of the type being measured. For each shot made, a bullet can be removed from the box, thus keeping the mass of the pendulum constant. The measurement change involves measuring the period of the pendulum. The pendulum is swung, and the number of complete oscillations is measured over a long period of time, five to ten minutes. The time is divided by the number of oscillations to obtain the period. Once this is done, the formula generates a more precise constant to replace the value 0.2018 in the above equation. Just like above, the velocity of the bullet is calculated using the