High Quality Content by WIKIPEDIA articles! The Oerlikon 20 mm cannon is a series of autocannons, based on an original designed by Reinhold Becker of Germany, very early in World War I, and widely produced by Oerlikon Contraves and others. Various models of Oerlikon cannon were used during World War II, and they are still in use today. During World War I, the German Reinhold Becker developed a 20 mm cannon, known now as the 20 mm Becker using the API blowback (Advanced Primer Ignition blowback) method of operation. This used a 20x70 RB cartridge and had a cyclic rate of fire of 300 rpm. It was used on a limited scale as an aircraft gun and an anti-aircraft gun towards the end of that war.