The S18 was originally designed in Germany by Rheinmetall and was further developed in Switzerland by Solothurn AG. In the early 1930s, Solothurn bought large shares in the Austrian factory of Waffenfabrik Steyr and the two companies merged into the Zurich-based Steyr-Solothurn AG; the S18 was produced through this joint firm to circumvent the terms of the Treaty of Versailles, which would have made production and export in Germany impossible.
The S18-100 was evaluated by the Swiss and German armed forces but was not adopted by either. However, Hungary adopted it as the 36.M in 1936 and produced under license at Danuvia. Bulgaria also adopted it the same year, and in 1938, the Tallinn Arsenal in Estonia began to produce unlicensed copies, known as the Solothurn-Arsenal. The Finnish Defence Forces adopted a modified version, known as the S18-154, in 1940, however only 12 were purchased and they were delivered late.
The US Army evaluated the S18-1000 in 1940-41, giving it the designation 20mm automatic gun T3. In 1941 tests against the .90 cal (23mm) Colt T4 aircraft cannon, the Solothurn performed better while also proving less bulky and complex, and a deal was reached for the US to procure 50 further examples. However, the long negotiations over rights to produce further guns in the US led to the abandonment of the entire project.
The S18 was recoil operated and fired a 20mm cartridge. It also had a recoilling barrel. Casings were ejected downwards, in a similar manner to the Finnish Lahti L-39.
The gun was extremely heavy and typically transported on vehicles or with the barrel detached: the enormous weight of the weapon did assist in mitigating the fierce recoil to an extent.
This was the smaller rifle, firing a 20x105mmB cartridge.
Aircraft cannon version of the S18-100.
This was a larger iteration of the S18, firing the 20x138mmB round used in the German 2cm FlaK 30 and 2cm FlaK 38 antiaircraft guns and the Finnish Lahti L-39 anti-tank gun. The magazine well dimensions are identical to the 2cm FlaK 30, meaning in theory it can use the 20-round magazine from that weapon. It can be identified from a S18-100 by the use of a crank rather than a charging handle.
Anti-aircraft version of the S18-1000 with the capability for full auto fire.
This is an unlicensed copy of the S18-100 produced in Estonia. Only 20 were produced prior to the Soviet invasion.