Original Item: 20mm Polston Oerlikon Barrel cal 20x 110RB. Used. Approximately 58 inches long.

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.

Because the treaty of Versailles banned further production of such weapons in Germany, the patents and design works were transferred in 1919 to the Swiss firm SEMAG (Seebach Maschinenbau Aktien Gesellschaft) based near Zürich. SEMAG continued development of the weapon, and in 1924 had produced the SEMAG L, a heavier weapon (43 kg) that fired more powerful 20x100RB ammunition at a slightly higher rate of fire, 350 rpm. In 1924 SEMAG failed. The Oerlikon firm, named after the Zürich suburb where it was based, then acquired all rights to the weapon, plus the manufacturing equipment and the employees of SEMAG.