Wikipedia: The STEN (or Sten gun) is a family of British submachine guns chambered in 9×19mm which were used extensively by British and Commonwealth forces throughout World War II and the Korean War. They had a simple design and very low production cost, making them effective insurgency weapons for resistance groups, and they continue to see usage to this day by irregular military forces. The Sten served as the basis for the Sterling submachine gun, which replaced the Sten in British service until the 1990s, when it, and all other submachine guns, were replaced by the SA80.
The Sten is a select fire, blowback-operated weapon and loads its magazine on the left, rather than the bottom, as is the usual configuration. Sten is an acronym, from the names of the weapon's chief designers, Major Reginald V. Shepherd and Harold J. Turpin, and "En" for the Enfield factory.[9][b] Over four million Stens in various versions were made in the 1940s, making it the second most produced submachine gun of the Second World War, after the Soviet PPSh-41.
Design: The Sten was a blowback-operated submachine gun firing from an open bolt with a fixed firing pin on the face of the bolt. This means the bolt remains to the rear when the weapon is cocked, and on pulling the trigger the bolt moves forward from spring pressure, stripping the round from the magazine, chambering it and firing the weapon all in the same movement. There is no breech locking mechanism, the rearward movement of the bolt caused by the recoil impulse is arrested only by the mainspring and the bolt's inertia. The German MP40, Russian PPSh-41, and US M3 submachine gun, among others, used the same operating mechanisms and design philosophy of the Sten, namely their low cost and ease of manufacture. Though the MP40 was also built largely for this purpose, Otto Skorzeny went on record saying that he preferred the Sten because it required less raw material to produce, and performed better under adverse combat conditions.[16] The effect of putting lightweight automatic weaponry into the hands of soldiers, greatly increased the short-range firepower of the infantry, especially when the main infantry weapon was a bolt-action rifle capable of only around 15 rounds per minute and not suited for short-range combat.[citation needed] However, the open-bolt firing mechanism, short barrel, and use of pistol ammunition severely restricted accuracy and stopping power, with an effective range of only around 100 m (330 ft), compared to 500 m (1,600 ft) for the Lee-Enfield rifle.