The Oehler Model 35BNC Chronograph is typically used to measure projectile velocities indoors. The chronograph, attached to screens such as the Oehler Model 57 Photoelectric Screens, measures the average velocity between two screens. Typically, the first screen or sensor is positioned 5 or 10 feet from the muzzle and the second screen is placed 10 or 20 feet beyond the first. Two screens are required to measure a single velocity. A third screen may be placed midway between the first two screens. If a third or middle screen is used, the unit records two velocities on each shot. The primary velocity is measured from first to last screen and a proof velocity is measured between first and middle screen. Both velocities are printed for comparison and verification, but only the primary velocity is summarized. The unit is powered from 120 volt ac converted to 9 volt dc by a wall mounted transformer power supply. The paper tape printer uses standard 2.25″ adding machine paper with a replaceable ink roller.