 It took several forms when applied to different aircraft types, but all shared the feature of a quadrant shaped I-beam rail on which the gun could slide back and down in one movement.The primary purpose was to facilitate the changing of spent ammunition drums, but some pilots also found that the mounting permitted the gun to be fired directly upward or at an angle, permitting a fighter aircraft to attack an opponent from beneath.Before the ready availability of a reliable synchronization gear, several methods were tried to mount a machine gun in a position from which it would be able to fire "past" rather than "through" the arc of the propeller.