The STEN (or Sten gun) was a family of British 9mm submachine guns used extensively by British and Commonwealth forces throughout World War II and the Korean War.. They were notable for having a simple design and very low production cost making it an effective insurgency weapon with resistance groups. STEN is an acronym, cited as derived from the names of the weapon's chief designers, Major Reginald V. Shepherd and Harold Turpin, and EN for Enfield. Over 4 million Stens in various versions were made in the 1940s. Sten guns were produced in several basic marks (though the Mk I saw limited service, and the Mk IV was never issued), and nearly half of the total produced were Mark II versions. Approximately 4.5 million Stens were produced during the war.