The Solothurn S-18/100 20 mm Anti-Tank Cannon was a Swiss and German anti-tank rifle used during the Second World War.
It had a semi-automatic action in a bullpup configuration. As a result of its large, powerful ammunition, the gun had a tremendous recoil, and its size made portability difficult. The feed was either from a five or (more usually) ten-round magazine that was attached horizontally to the left side of the gun. The gun used 20×105mm belted-case ammunition which it shared with the S 18-350 aircraft cannon that was developed from the rifle. A Finnish source gives armour penetration of the gun (probably achieved with the Hungarian APHE-T round, since it was the only type used in Finland) as 20mm at a 60-degree angle at 100-metre distance, decreasing to 16mm at 500 metres.
In March 1940, with funds collected in Switzerland to help the Finnish war effort in the Winter War, Finland bought twelve S 18-154 anti-tank rifles from Solothurn, though the purchaser was nominally the Swiss army. The weapons arrived into Finland during the spring after the war had ended, but they were later used in the Continuation War. However, the guns were soon found to be obsolete in their intended role. Various models of the S-18 series, including the Solothurn S-18/1000 and the Solothurn S-18/1100 were also used by Switzerland, Hungary, Germany, Italy, and the Netherlands.
The Solothurn firearms company was owned by the German firm Rheinmetall, and used the Swiss company to manufacture arms which were prohibited for manufacture by any German firm, to get around arms limitations imposed upon them at the end of the First World War.