The MG 34 was based on a 1930 Rheinmetall design, the MG 30. The Swiss and Austrian militaries had both licensed and produced the MG 30 from Rheinmetall shortly after patent and started to enter service in Switzerland. In the spring of 1931 the development of the "Einheitsmaschinengewehr" (Universal machine gun) started. The MG 30 design was adapted and modified by Heinrich Vollmer of Mauser Industries. Vollmer originally designed the feed mechanism to accept MG 15 inspired 75-round "Patronentrommel 34" spring-loaded saddle-drum magazines. The "Patronentrommel 34" was a rather complex magazine for which a filling device existed and requiring ordnance personnel and a special tool to optimize the spring tension for reliable feeding. Users were ordered not to adjust the drum spring tension.