Different Sten gun models. Click on the image to enlarge.
First, we must go back in history to Europe in the summer of 1940. German soldiers were sweeping through Belgium and France and allied troops were in a desperate situation and trapped in the tiny port of Dunkirk. The British deployed every boat and ship available to rescue the stranded Allies and in nine days (27th May - 4th June), over 300,000 soldiers (British, French, Polish, Belgian, Dutch etc.) were evacuated to England. However, this rapid evacuation also resulted in soldiers leaving their equipment behind and large amounts of firearms fell into the hands of the Germans. Shortly after that, the Battle of Britain started and many factories in England were bombed. As a result of all this, there was a shortage of small arms in Britain. The British were buying Thompson submachine guns from the United States, but the factory could not keep up with the demand (and after 1941, many of those Thompsons went to the US military, so they couldn't supply anyone else anyway). Therefore, a decision was made to design a submachine gun that could be made in England quickly and cheaply.
The task of designing this new weapon fell to Major R.V. Shepherd of the Design Department at the Royal Arsenal, Woolich and Mr. Harold J. Turpin, of the Design Department of the Royal Small Arms Factory, Enfield. The design they came up with was called the STEN. The "S" and "T" in the name came from the first letters of the designers last names (S from Shepherd and T from Turpin) and the "EN" came from the first two letters of "Enfield".
From the beginning, the aim was to design a cheap gun that could be manufactured with a minimum of machining operations. It had to be capable of being manufactured in small workshops and produced as quickly as possible. It also had to be capable of single shot and automatic fire and designed for close range fighting. It was designed to use the 9x19 mm. Parabellum Luger cartridge, which was also used by the Germans. The Sten was also deliberately designed to fit German 9 mm. magazines from the MP-38 and MP-40, so that they could use captured German ammunition and equipment if needed.
The design that they came up with was a submachine gun using a blowback mechanism and firing from an open bolt. When the weapon is cocked, the bolt remains at the rear of the weapon. When the trigger is pulled, the bolt is pushed forward by spring pressure and strips a cartridge from the magazine, chambers it and then fires it. The firing pin is fixed in front of the bolt. After the cartridge discharges, the bolt moves rearward against spring pressure and inertia of the heavy bolt and then recocks itself. The working components of this weapon are housed in a basic tubular metal receiver with a barrel on one end and a wire shoulder support welded to the other end, with a simple trigger mechanism in between
The Sten gun, Mark I
The first version of the Sten gun, the Mark I model, came with a conical flash hider and contained some wooden parts (the foregrip and part of the stock). The front pistol grip could also be rotated to make the firearm smaller and therefore, easier to pack. Production started in late 1940 and about 100,000 of this model were made.
Compared to the Mark I model, the Mark II model was much more stripped down. The flash hider was removed and the folding front pistol grip and all the wood were eliminated as well. This made the Mark II smaller and lighter than the Mark I model.
The Mark II variant was the most commonly manufactured model and about 2 million of these were produced. Some Mark II models were made with integral suppressors attached and were classified as Mark II (S)
The Mark III variant was even more stripped down than the Mark II model and was first produced in 1943. In this model, the receiver and the barrel shroud are made from a single tube, by wrapping a sheet steel plate into a cylindrical shape and welding the top. This model is also a bit lighter than the Mark II model.
The Sten Mark III model was the second most commonly produced model of the Sten gun family and was the most stripped down model of the series, and therefore the lightest version.
By 1944, the threat of a German invasion of Britain was over and the Sten gun quality improved. Models Mark IV and Mark V had better quality fit and finish and even came with wooden parts.
The Sten gun model Mark V
The Mark IV model was a paratrooper's model with a folding stock, but never got off the prototype stage. The Mark V model had better sights and finish and came with a bayonet attachment as well.
The Sten was designed to be manufactured quickly and easily. This is why most of the components could be manufactured by stamping sheet metal and doing some minor welding. From the beginning, many of the parts were subcontracted to small workshops, with final assembly being done at the Enfield factory. This was especially useful as the larger factories were being bombed from the air by the German Air Force, early on during the war. The design was made simpler with each generation and the Mark III model only had 47 parts. Interestingly, one of the largest manufacturers of the Mark III model was a toy company called Lines Brothers. The Sten was really cheap to manufacture and only cost about $10 to make, which was much cheaper than the Thompson submachine gun, which cost about $200 then.
While Sten guns were cheap to manufacture, they occasionally had jamming issues as well. The gun was designed to use the same magazine as the German MP-38/MP-40, so that people could reuse captured equipment. However, it also inherited the problems of the German magazine, in particular dirt could cause it to jam. In the absence of a pistol grip and forward grip in the Mark II and Mark III versions, some soldiers would hold the magazine with the supporting hand, causing it to wear out the magazine catch and cause failure to feed issues. The safety device was rudimentary and there was a danger of accidental discharge upon dropping the weapon, especially since many were crudely made. The Mark V model attempted to fix some of these issues.
The Sten was loved and hated by its users at the same time. Many didn't like its peculiar appearance and reliability (at least for the Mark II and Mark III models) and it was nicknamed the "Plumber's Nightmare" and the "Stench Gun". However, they liked its cheap cost and short range firepower. It was manufactured during World War II by many British companies, as well as workshops and factories in Canada, Australia, France, Poland, Denmark, Norway etc. It was responsible for the US manufacturing its own cheap submachine gun model: the M3 grease gun. Towards the end of World War II, even the Germans got in the act and made over 28,000 copies of the Sten gun. After World War II, many were made in small workshops Israel in 1948. The Sten gun is still in use in some countries around the world.