Introduced in 1944, the Mk V was essentially a better-quality, more elaborate version of the Mk 2. However, the open-bolt firing mechanism, short barrel, and use of pistol ammunition severely restricted accuracy, with an effective range of around 100m. These shared similar attributes and faults they were simple and cheap to manufacture, and put an automatic weapon into the hands of soldiers, greatly increasing 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. The basic operating principles were like those of the German MP40, Russian PPSh-41, US M3 submachine gun and numerous other designs. 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. This means the bolt remains to the rear when the weapon is cocked, and on pulling the trigger the bolt moves forward under spring pressure, stripping the round from the magazine, chambering it and firing the weapon all in the same movement. The Sten is a blowback-operated submachine gun firing from an open bolt with a fixed firing pin on the face of the bolt. They had a simple design and very low production cost, so they were also effective insurgency weapons for resistance groups. The STEN (or Sten gun) was a family of British submachine guns chambered in 9Г—19mm and used extensively by British and Commonwealth forces throughout World War II and the Korean War.

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