The design was based on two of the most popular submachine guns of the era. The external construction with a retractable butt and magazine mounted under the gun was borrowed from the successful German MP 40. The internal design of the mechanism was modeled after the British Sten, Blowback, with an open bolt. It offered good performance and high reliability. The weapon was designed so that resistance army members could use any captured stocks of German MP40 ammunition cartridges. That’s right, ammo they took from the Germans, not necessarily in a friendly fashion.