The Solothurn ST-5 is a 20 mm anti-aircraft gun designed by Waffenfabrik Solothurn in Switzerland which ultimately was the design basis for the highly successful 2 cm Flak 30 series of guns used by Germany in World War II. Solothurn was owned by the German firm Rheinmetall, which at the time was prohibited from developing certain weapons. It and other German manufacturers sought partnerships with foreign firms, often owning them outright, in an effort to circumvent the terms of the Versailles Treaty. The gun was initially developed by Germans engineers employed in a Dutch front company of Rheinmetall, Hollandsche Industrie- en Handelsmaatschappij (H.I.H., informally also "Haiha") which is better known as HIH Siderius. The ST-5 fired the 'Long Solothurn' ammunition, the cartridge 20 × 138 mm. B. The cartridge case was 138 mm long, making it the most powerful 20 mm round available.

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