After the British Expeditionary Force’s retreat from Dunkirk in June 1940, Germany was expected to attempt a full-scale invasion of Britain. In order to accomplish this, they would need to eliminate the Royal Air Force, and as such, the RAF’s airfields were at serious risk. The RAF wanted a Schmeisser-type submachine gun issued to their personnel in the event of an attack from German paratroopers. The Navy had already ordered 2000 Smith & Wesson 9mm carbines and the Army, who by now had realized that they had vastly underestimated the military effectiveness of the submachine gun, began buying Thompsons from the United States.
The RAF initially examined captured MP-38s and ordered 10,000 British-made copies, but there were complications that resulted not only in the weapons being changed from copies of the MP-38 to the MP-28, but also the order being increased to 50,000 to satisfy the Navy as well, who had been forced to abandon the S&W carbines due to serious malfunctions. Sterling Armaments Co. was contracted to produce the initial prototypes of the MP-28 copy. The resultant weapon was finished, in the form of two pilot guns, in late 1940 and demonstrated on the 8th of November. The pilot guns were designed by George Lanchester and thus were named after him.
The Lanchester pilot guns were tested again on the 13th of November 1940 and were tested with a variety of 9x19mm catridges, including Winchester flat-nose, ICI, Bergmann, Beretta, and German military issue. The first pilot gun failed to discharge the Winchester and ICI ammunition, but the second did not run into any major issues and was considered on-par with the German MP-38.
On November 28th, further trials of the Lanchester pilot guns took place in the presence of both George Lanchester and Major Reginald V. Shepherd of the Design Department at RSAF Enfield. The Lanchester was now in its fourth pilot gun form and fired 5204 rounds with 26 stoppages. It passed all the mandatory tests but did not function when loaded with Beretta-made ammunition. Otherwise it was considered good to go and production rights were handed over to the Royal Navy for immediate manufacture as the Lanchester Mk.I. This weapon was issued to the Air Force and Navy until 1941, when it was simplified as the Mk.I*, which had no fire selector and fixed iron sights.
The Lanchester was good but production costs were too high to equip the army. Something cheaper and quicker to manufacture was sought. In January 1941, an extremely simplified model was designed by George Lanchester and demonstrated at Enfield on the 10th of January 1941, and at Hythe on the 21st. The prototype was essentially a Lanchester stripped down to the bare minimum. It consisted of a simple tubular body made from steel and grips made from Tufnel. It was supposed to have a folding buttstock but for whatever reason this was never fitted. The only real change to the base design was the inclusion of a fire selector just in front of the trigger grouping. Otherwise it was internally the same as the Lanchester Mk.I.
A second simplified prototype was also conceived by George Lanchester and differed in that the cocking slot was now on the left side of the gun and had a much lighter bolt which was about an inch shorter than the original. The grips were redesigned to be more ergonomic, and a simple single-strut stock was fitted to the rear of the pistol grip.
Both simplified prototypes of the Lanchester were tested but rejected. But from this concept, the Sten was born. It was developed in early 1941 by Major Shepherd and Harold J. Turpin, who worked at the Design Department at Enfield. Thus the weapon was christened the STEN (Shepherd, Turpin, ENfield). The design was an incredibly simple blowback system based on the Lanchester with a fixed firing pin and simple cylindrical bolt. The first version of the Sten, the Mk.I, had wooden furniture, a conical flash hider, and a hinged fore grip, a feature not seen on any of the subsequent models. The Mk.I was cheaper than the Lanchester but still too expensive; it was simplified further as the Mk.I* in late 1941. The Mk.I* ditched the wooden embellishments, the flash hider and the fore grip feature. Throughout 1941, over 100,000 Mk.I and Mk.I* Stens were produced and issued to the army.
In mid-1941, the Mk.II Sten was designed. It was a bare-bones version of a gun which was already very basic. The main difference between the Mk.I and the Mk.II Stens was that the Mk.II had a new barrel that could not be interchanged with the original Mk.I barrel. The Mk.II barrel had only two grooves whereas the Mk.I had six. Externally, the Mk.II was incredibly minimalist. There were two main versions of the Mk.II produced: one with a wireframe stock and one with a single-strut stock. Neither were particularly pleasant to shoot, owing to the poor ergonomics. The upshot of all this was that the Mk.II Sten was incredibly cheap to produce en masse for the army and, as an added bonus, proved very easy for anti-Nazi partisans to copy in workshops.
The Mk.II Sten was tested at Pendine on from the 7th to the 25th of August 1941 and a glaring fault was discovered. The magazines were made from stamped sheet metal, which meant that the feed lips were prone to failure. If the magazine feed lips were misaligned even slightly with the magazine well, the gun would jam. The magazines were also highly susceptible to dirt and sand. All of this basically meant that the Mk.II Sten was highly unreliable if not handled with care, and even then it was probably inevitable that it would fail at some point during the heat of battle. But the army was faced with a choice between a mass of unreliable Mk.IIs, or a handful of Thompsons, Lanchesters and Mk.I Stens. They opted for the former.
The Mk.II Sten was by far the most successful model of the Sten gun, with over 2,000,000 being produced throughout World War II. It was first issued to British and Canadian troops during the raid on Dieppe on the 19th of August 1942 and continued to be issued until 1945. It was also issued in considerable numbers to the Free French Forces, including the French Resistance.
In 1943, the toy manufacturer Line Brothers Ltd. were contracted to produce the Mk.III Sten, which was made from a single, riveted sheet metal tube that was welded at the top. The ejection also had an extra safety precaution that consisted of a simple finger guard. The barrel was fixed inside the tubular body, which could not be disassembled. In Canada, the Mk.III was manufactured by Long Branch Arsenal.
The Mk.IV was the only one of the Sten “marks” not to be issued to the army. In fact, it never evolved past the prototype stage. It was designed in 1943 with paratroopers in mind, with a shorter barrel and folding stock. The first version of the Mk.IV had a conical flash hider and a very unusual pistol grip and trigger guard arrangement that was designed to facilitate for thick winter gloves. It was a mere 27 inches in length. After it was trialed at Pendine at rejected for improvements, a second version known as the Mk.IVB was developed which was designed to be fired with one hand. To achieve this, the balance of the weapon was changed by moving the trigger grouping forward to the middle of the gun. The trigger mechanism had to be completely redesigned to allow this. It was 24 inches in length but uncomfortable to fire. Besides its flaws, there was no immediate requirement for the Mk.IV model so it was never developed any further.
In 1944, the Mk.V Sten appeared. It was a much more presentable weapon and a far cry from the crude Mk.II The Mk.V featured a wooden butt, pistol grip and fore grip. The fore grip was ditched in later models. The front sights were also redesigned and lifted from the No.4 SMLE service rifle. Internally, the bolt was improved with a cutaway that cleared the trigger disconnector when the bolt came over the sear. The resultant weapon was of excellent quality and made to a much higher standard than its precursors. Unfortunately, cheaply-made magazine were still being issued and consequently the Mk.V was still just as liable to failure as the earlier models, although this was not the fault of the gun itself.
The Mk.V Sten was issued extensively to paratroopers after D-Day and saw considerable use during Operation Market Garden in Arnhem, and issue of the Mk.V continued until the war in Europe ended in May 1945.
Many variations of silenced Sten guns were also developed. British interest in silenced weapons began in 1940 when British Commandos demanded a quiet gun for eliminating lone sentries during covert raids. Initially they were issued silenced Thompsons made by RSAF Enfield, but these were too heavy and expensive to deploy in any numbers. When the Sten Mk.II appeared, Enfield developed a suppressed model called the Mk.IIS. It was designed by a Polish exile who was now serving with the Special Operations Executive, Lt. Kulikowski. The suppressor consisted of a series of metal cups wrapped around and in front of the barrel, with a rubber plug at the end. When the weapon was fired, the gases seeped out the sidewall of the barrel and their energy dissipated. The bullet traveled through the metal cups and penetrated the plug, which prevented the gases from escaping. These metal cups were encased in a perforated jacket which was surrounded by an additional jacket.
The Mk.IIS was issued to Commandos, the SOE, and other British special forces units, as well as resistance fighters across Europe. It was designed to be fired in single shots. Reportedly, the sound of the bolt was louder than the gunshot itself. The main drawback of the Mk.IIS was that it had an effective range of only 100 meters.
Late in the war, the Mk.V Sten was successfully silenced using a similar principle and this model was called the Mk.VI. It did not see as much use as the Mk.IIS but was probably, all factors considered, the best silenced weapon of the war. It was succeeded by the Sterling L34A1 silent submachine gun.