The STEN (or Sten gun) is a British submachine gun chambered in 9×19mm which was used extensively by British and Commonwealth forces throughout World War II and during the Korean War. The Sten paired a simple design with a low production cost, facilitating mass production to meet the demand for submachine guns. As well as equipping regular units, the Sten was distributed to resistance groups within occupied Europe. Its simple design made it an effective insurgency weapon for resistance groups. The Sten is a select fire, blowback-operated weapon with a side-mounted magazine.
https://en.wikipedia.org/wiki/Sten