The STEN (or Sten gun) was a British 9 mm submachine gun used extensively by British and Commonwealth forces throughout World War II. The gun was notable for having a simple design and very low production cost, making it effective insurgency weapons for resistance groups. STEN is an acronym, from the lastnames of the weapon's chief designers, Major Reginald V. Shepherd and Harold Turpin, and EN for Enfield.