In the late summer of 1940 Britain was in a desperate position. France and Norway had fallen in June, the Axis powers had surged across Europe destroying all opposition before them. Britain's armed forces were in disarray, the British Expeditionary Force had only barely survived its exodus from France and disastrous operations in Norway. The Royal Navy was struggling to cope with German U-Boats harassing the Atlantic supply convoys which had quickly become Britain’s lifeline. The British Army of late 1940 was in a dire situation, it had lost the majority of its heavy equipment during the retreat and evacuation from Dunkirk, equipment ranging from tanks to the average foot soldier's Lee-Enfield rifle, the number of which left behind varies - with possibly 100,000 rifles lost.Â
The Army was desperately trying to rearm the remnants of the B.E.F and also equip newly mobilised troops.  The British had learnt effectiveness of the German MP40 submachine gun the hard way in France, it was decided a submachine gun was needed urgently. The War Office and Government had authorised the purchase of every available Thompson Submachine gun they could get their hands on, but with transport time across the Atlantic and the increasing need for new small arms the War Office looked to the Royal Small Arms Factory at Enfield for a British, home-grown weapon to compliment the Thompsons.  Â
In the winter of 1940, a design team including Harold Turpin and Major R.V. Shepherd convened to design a new weapon which could be mass produced quickly and cheaply. In just 36 days the design went from the approval of the weapon's concept to the first working prototype being made. Designed chiefly by Turpin, the first hand built STEN was made in December 1940 at a small workshop in Middlesex. It was designated the T40/1 (Turpin 40), this first model is today held by the British Army's Small Arms School museum (see below).Â
T40 Prototype, not the barrel shroud perforations for cooling, these were later eliminated (source)
The first STENs looked radically different from what immediately comes to mind for most of us, from a distance at least, it looks more refined with a wooden pistol grip integrated into many early models and a wooden fore-stock and folding hand-grip.  However as you can see from the close up images above the workmanship was rough and ready with the stamped parts quickly welded together. This was a stark departure from the careful, high-quality standards of pre-war British infantry weapons and even the contemporary Lanchester submachine gun.
The STEN used a blowback, open-bolt mechanism similar to that of the German MP40, Russian PPSh-41 and the later STEN-influenced American M3. The STEN fired the readily available 9mm Parabellum round, which would enable troops in the field to also use captured enemy ammunition - unlike those issued with a Thompson which was chambered for .45 ACP. The trigger mechanism designed by Turpin was the model of simplicity and was the basis from which the STEN was developed. The MkI had just 59 component parts, this would later be reduced with subsequent models.
Named after its main designers Major Shepherd and Turpin and the factory where it was assembled, Enfield the weapon quickly became known as the S.T.EN Gun. Officially it was classified as a 'machine carbine' by the British Army. The first batch of MkIs (as seen above) were shipped in mid 1941, with around 100,000 of the first model being manufactured before an even simpler design replaced it.
The MkI was quickly succeeded by first the MkI* and then the instantly recognisable and most widely produced MkII. The subsequent developments of the STEN stripped back almost all refinement and complexity to the point where a STEN could be build in as little as 15 hours and cost as little as £2. A truly remarkable weapon which began its life as a stopgap measure but went on to serve the British Army reliably for 25 years and spawned half a dozen imitations from countries around the world.  Â
Machine Gun, Ian V. Hogg (1979)
The Sten Gun, L. Thompson, (2012)
Firearms Anatomy: The Sten Submachine Gun, D.S. Findlay (2013)
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