Because of the restrictions placed upon it following its defeat in World War 1 (1914-1918), the nation of Germany was forced to be creative in its rearmament schemes heading into World War 2 (1939-1945). The Treaty of Versailles prohibited many major weapon types such as submarines and tanks outright while more minor ones, like submachine guns, were restricted to design only. As such, several weapons born in Germany were developed and manufactured elsewhere. This proved the case with the S1-100 submachine gun which went on to be best remembered as the "MP34".
Rheinmetall of Germany headed the design a this new submachine gun which was in essence a further evolution of the World War 1 Maschinenpistole Model 1918 - the "MP18" - which became the world's first true submachine to be adopted in number by a first-rate army. Rheinmetall then sent its preliminary work to its subsidiary company in Switzerland - Waffenfabrik Solothurn AG - where the design could be prototyped and trialed. However, Solothurn lacked the production facilities to build the new weapon in the numbers required and its national rules restricted weapons exports. This ultimately led to the submachine gun being sent to Waffenfabrik Steyr of Austria where it could be made as the Steyr-Solothurn "S1-100". Production began in 1929.
The S1-100 found favor with Austrian police forces and was adopted as the "Steyr MP30". These weapons used the standardized Austrian 9x23mm Steyr pistol cartridge. The Austrian Army followed suit when it took the S1-100 as the "Steyr MP34" though these guns were chambered for the 9x25mm Mauser (Mauser Export) cartridge.
The S1-100 was constructed to a high standard which made it both heavy and expensive to mass-produce. However, these qualities also made for an astoundingly efficient automatic weapon. It fired from a blowback design with an open bolt arrangement out to ranges of 650 feet (effective maximum). The system could sustain a rate-of-fire of 500 rounds per minute. Sighting was through an adjustable rear assembly and hooded (or open) front post. The gun's body featured a rifle-style wooden receiver with integral shoulder stock and grip handle while the key metal components were inlaid. As in other submachine gun designs of the period, the 20- or 32-round detachable box magazine was inserted into the left side of the weapon. The barrel protruded only a short distance ahead of the wooden body and was shrouded in a heavily perforated jacket to promote both cooling and operator safety. The barrel also held a bayonet mount for close-in work by blade if need be. Sling loops allowed a shoulder strap to be used and facilitate transport over the shoulder. Overall weight was 9.9lb with an overall length of 33.5" and 7.9" long barrel.
The S1-100 was also showcased to Asian, African, and South American buyers where they proved popular weapons. Several differently chambered versions emerged such as the .45 ACP cartridged form seen in South America which also added a forward grip handle under the stock. Other versions seen fired the 7.63mm Mauser or 7.65mm Parabellum cartridges. The submachine gun was purchased in quantity by China while also being adopted closer to home in neighboring Hungary and Portugal by their respective armies. The Chinese used the guns with good effect against the Japanese who also were not shy about using captured guns on the original owners.
When Nazi Germany annexed Austria in 1938, large stocks of the guns fell to the Germans. These were reworked to fire the German-standard 9x19mm cartridge instead and carried the designation of MP34(o) to mark their Austrian origins. Austrian lines continued to produce the submachine gun during World War 2 until about the middle of 1940 to which manufacture then shifted to the cheaper German MP40 series - the standardized submachine gun of the Wehrmacht to the end of the war. In the final years of the conflict, the MP34 was primarily in the hands of second-line forces.
Many variants of the original MP18 design eventually appeared - some with direct lineages dating back to the original weapon and others that simply copied its best qualities.