The birth of the storied Swiss Oerlikon 20mm automatic anti-aircraft cannon was actually in Imperial Germany when it was debuted for service in World War 1. The weapon was from a design by Reinhold Becker in 1914 and came to be known in its original form as the "20mm Becker", an automatic firing weapon chambered for the 20x70mmRB projectile and intended for use by the German Air Service. Despite this early initiative, the weapon came to be featured in a variety of roles and is best remembered as a fixture on many warships. After the war ended with the Armistice of 1918, manufacture of the gun was moved to neighboring Switzerland in 1919 due to the restrictions of the Versailles Treaty which severely limited German war-making capabilities. The brand label responsible for the weapon was now SEebach Maschinenbau Aktien Gesellschaft (SEMAG).
Along with the Swedish Bofors 40mm weapon, the Swiss Oerlikon 20mm ranks as one of the most important cannon developments of the 20th Century - being utilized in land-based (fixed or towed), naval platforms, and as aircraft armament. For the naval role, single-gunned, twin-gunned, and even quadruple-gunned mounts were eventually realized. The weapon proved a fixture on the classic warships of World War 2 - from the smaller patrol boats to the massive battleships and aircraft carriers featured in the conflict. The automatic cannon was fielded by all sides of the war including the United Kingdom, France, the United States, Germany, Italy, and Japan.
At its core, the weapon relied on a gas-operated, blowback action (with Advanced Primer Ignition - API) to produce its automatic firing effect. The recoil springs were wrapped around the barrel and helped to improve the repeat fire accuracy needed. Feeding was originally by way of a 20-round box magazine or a 60-round drum type though the latter prove the more widely accepted method. A High-Explosive (HE) shell was the standard projectile and these could range out to 7,400 yards (absolute maximum) though direct target firing tailed off after about 1,600 yards.
SEMAG introduced a new model, the SEMAG L, in 1924 which incorporated a faster rate-of-fire through a heavier frame and was chambered for the 20x100mmRB projectile. However, this addition to the product line was not a success. The product was eventually absorbed as a part of Werkzeug Maschinenfabrik Oerlikon located in the north Zurich and the type's reach only grew after this period as local license production rights were granted to several world powers where they took on local designations.
The next major mark to appear was the Oerlikon S model of 1927. This product retained the 20x110mmRB chambering but saw its muzzle velocity raised for better penetration value allowing it to be used in a dual-purpose role (against both air and land targets). The Oerlikon 1S followed in 1930 and was categorized as an improved version. The Oerlikon AF and Oerlikon AL models were introduced as dedicated aircraft weapons to be fitted on trainable mountings and a 30-round drum was introduced for this application.
A fixed aircraft mounted version debuted in 1935 as the faster-firing Oerlikon FF (firing at about 520 rounds per minute) and variable ammunition drums were offered that spanned from 45 to 100 round counts. This model was chambered for the 20x72mmRB projectile. The localized German version became the famous MG FF cannon fit though chambered for 20x80mmRB projectiles. The FF in Japanese naval service became the Type 99-1. The similar Oerlikon FFL had its rate-of-fire reduced (to 500rpm) but fired a heavier 20mm projectile. The Japanese naval equivalent became the Type 99-2. The heaviest projectile fired came from the Oerlikon FFS model which saw a further reduction in rate-of-fire (470rpm).
The Oerlikon FFS was evolved in 1938 to become the classic naval Oerlikon SS which featured a 650 rounds-per-minute rate-of-fire. The SS model became the naval standard of World War 2 fighting. During 1942, the Oerlikon 1SS debuted and, in 1945, the Oerlikon 2SS model followed - both products attempting to increase rate-of-fire values.
Amazingly, the Oerlikon product still remains in service today (2015) despite its original showing in the Great War. Such longevity for a military weapon displays the design's excellent internal function, battlefield versatility, and tactical appeal. Modernized forms will only continue the line that has produced a "100-year weapon" - something of a true rarity in the realm of military service.