In 1930, the Austrian police accepted the S1-100 as the Steyr MP30, chambered for then standard Austrian 9 x 23 Steyr pistol rounds. The weapon was also exported to Chile, Bolivia, El Salvador, Uruguay and Venezuela, and was sold in limited numbers to China, in 7.63x25 Mauser calibre.
For the South American markets, Steyr produced a version of the S1-100 in .45 ACP calibre; this derivation can be identified by an additional pistol grip under the stock.
The Austrian army adopted the Steyr-Solothurn S1-100 as the Steyr MP34, chambered for the powerful 9×25mm Mauser ammunition.
With the 1938 Anschluss between Germany and Austria, the German Army acquired most of available MP30s and MP34s. A number were then re-barrelled to chamber 9×19 ammunition and issued to German troops as the MP34(ö) - Maschinenpistole 34 österreichisch (literally "Machine-pistol 34, Austrian"). Production of the MP34 ceased in mid-1940, and manufacturing lines at Steyr moved over to the production of the MP40 – a much simpler designed weapon and far less expensive to produce than the MP34. As a substitute standard small arm, it had a relatively short combat service once quantities of the MP38 became available, though some MP34s were used by Waffen SS units in the early stages of the war in Poland and France. It was then allocated to line-of-communications and reserve units, including military police and Feldgendarmerie detachments.
In Greece, various police forces under the Ministry of Security, notably the mechanized police, were equipped with the MP34 in 9×25mm Mauser caliber.
Portugal bought in small quantities the .45 ACP version and was adopted as Pistola-metralhadora 11,43mm m/935. Portugal also purchased small quantities of the S1-100 in 7.65x21mm Luger calibre in 1938, and the weapon was adopted as the Pistola-metralhadora 7,65 mm m/938 Steyer submachine gun. In 1941 and 1942, larger numbers of 9mm MP34 guns were delivered to Portugal by Germany. In Portuguese service, the 9mm MP34 was known as the Pistola-metralhadora 9 mm m/942 Steyer. Many m/942 guns carry a Portuguese crest just forward of the safety mechanism in combination with Waffenamt (WaA) markings. The m/942 remained in service with Portuguese Army into the 1950s, and was used until the 1970s by paramilitary and security forces in Portugal's overseas African colonies during the Portuguese Colonial Wars.
The MP 34 was also the most common issued submachine gun of the Imperial Japanese Army and Navy before and during the Pacific War.