As effective as the Swiss-made Oerlikon 20mm automatic cannon (detailed elsewhere on this site) proved to be for many heading into World War 2 (1939-1945), it was a complex and expensive weapon to produce for some global operators. A team of engineers in Poland took up the task of redesigning the weapon system into a more economical product resulting in the gun that would be called the "POLSTEN". While largely a Polish development at its core, its primary operators and manufacturers during the conflict became Canada and Britain.
The POLSTEN name is thought to have come from the combined words "Poland" and "STEN".
The POLSTEN project was nearing its end in September 1939 when the German Army rolled over Polish borders. Before defeat had set in, the engineering team fled with their work to the relative safety of England where they continued the project, now with support from local talent as well as assistance from relocated Czechs. The modifications to the Oerlikon design including a massive reduction in parts - up to 47% - and this not only simplified production and maintenance and repair but also lowered procurement costs. Beyond supporting a 20mm ammunition drum (60-rounds), the POLSTEN was given the ability to feed from a 30-round, top-mounted box magazine (double-stacked). The chief victory in the revision work was that the weapon lost none of the Oerlikon's effectiveness in-the-field.
With the project completed and evaluations had, the POLSTEN was quickly pushed into service during the spring of 1944 and Canadian factories participated in its serial manufacturing (John Inglis Ltd, Ontario). While Allied production of Oerlikons was given up in favor of the cheaper, simpler POLSTEN, the Polish gun never superseded the original in terms of numbers available and battlefield reach. In service, the POLSTEN was used as a local air-denial weapon against low-flying aircraft and featured as a towable, lightweight AA gun system and well as in double, triple and quad cannon mountings atop military trucks and trailers. Some Commonwealth tanks also featured the gun as secondary armament.
Atop its lightweight 'Universal Mounting', the POLSTEN had an overall length of 7 feet and weighed 121 lb. The mounting hardware allowed for an elevation span of -5 to +85 degrees to be reached. Traversal was a full 360-degrees. The 20mm projectiles exited the gun at a muzzle velocity of 2,725 feet per second. Rate-of-fire reached 450 rounds per minute. The gun held a reach out to 6,630 feet of altitude.
In the field, there was no reduction of efficiency and effectiveness for the Polish derivative of the Oerlikon and its versatility shown through by way of its many applications. Despite this it never took over the Oerlikon on a global scale simply due to the Swiss gun's availability.