The Sten Gun

In our last post, we saw how America adopted the M3 a.k.a the Grease Gun. In today's post, we will look at one of the guns that inspired it, the British Sten gun. This was a gun that was designed to be manufactured cheaply and easily and we will study its origins and design today.

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.

The M3 Grease Gun

In today's post, we will study a submachine gun that was in US service for 50 years and is still used by some military forces elsewhere in the world. Today's post will study about the famous M3 and M3A1 submachine guns, more popularly known as "grease guns".

The origin of this design has to do with the onset of World War II. In Europe, the Germans had developed the MP-40 submachine gun and the British had the Sten submachine gun. Both these weapons were chambered to fire the reliable 9x19 mm. Luger cartridge. In 1941, the US Army Ordnance Board conducted some studies on the effectiveness of these weapons on the battlefield and determined that there was a need for similar weapons for the US military as well. At that time, the US military had already adopted the Thompson submachine guns (which were developed at the end of World War I) into service in 1938, but the Tommy guns were relatively expensive to manufacture. The Ordnance Board wanted a weapon that could fire the same .45 ACP cartridge of the Thompson, but which could be manufactured much cheaper and could be fired as accurately as well.

The final list of requirements included:

1. Weapon to be made of metal completely, with largely sheet metal construction, in order to speed up manufacturing.
2. Weapon designed to fire .45 ACP cartridges, since the US military was already using this cartridge (The Germans and British were using 9 mm. Luger cartridges for their submachine guns)
3. Weapon to be designed for fast production, with a minimum of machining operations.
4. Weapon to be designed to be cheap to manufacture (cheaper than the Thompson submachine gun)
5. Weapon to be designed as reasonably accurate. It was required to demonstrate that this weapon could put 90% of shots fired in a standing position in full-automatic mode into a 6x6 foot target at a distance of 50 yards. 
6. Weapon to be capable of firing in both semi-automatic and full-automatic modes. This requirement was later removed during development of the weapon and it was designed to fire full automatic only.

The task of designing this weapon fell to George Hyde of General Motors' Inland Division. George Hyde was actually of German descent and had emigrated to the US in 1927, whereupon he changed the spelling of his last name from "Heide" to "Hyde". He was a well known gun designer in 1941 and was put in charge of designing the new weapon. Meanwhile, another engineer named Frederick Sampson was put in charge of preparing the tooling for mass production of this weapon. During the design phase, a conversion kit was also designed, which would allow the user to quickly convert the gun from firing .45 ACP to firing the 9 mm. Luger cartridge.

The prototypes were completed in late 1942 and approved for mass production. The Guide Lamp division of General Motors (the same people that manufactured the FP-45 Liberator pistol) was put in charge of manufacturing it. While the weapon was given the official designation of "U.S. Submachine Gun, Caliber .45, M3", many people noticed that it resembled grease guns used by automobile mechanics and therefore, the new weapon was nicknamed the "Grease Gun".

The M3 Grease Gun. Click on the image to enlarge.

Image licensed under the Creative Commons Share-Alike 3.0 Unported license by Curiosandrelics

The M3 was designed to be cheap. The manufacturing cost of one of these in World War II was around $18. Only three parts, the bolt, the barrel and the firing mechanism, were precisely machined. Most of the other parts of this gun were manufactured by using metal stamping and pressing technologies. Spot welding and seam welding were used to join most of the parts together and some other parts were riveted together. These processes allowed the gun to be manufactured very rapidly and with low cost. The stock was simply made from a single steel rod, which was bent into shape. The ends of the stock were drilled and tapped, so that it could be used as a cleaning rod, as well as a disassembly tool. In the M3A1 model, the stock also had a tool welded to it, to be used to load a magazine. The safety was a projection on the inside of the ejection port dust cover, which locked the bolt into the forward or rear positions. However, since the metal around the dust cover was so thin, it could get bent easily and the safety mechanism wouldn't work any more.

The two halves of the receiver were made by stamping sheet steel and then the two halves were welded together. The bolt was made heavy and made to run on two parallel guide rods, which had twin return springs. This allowed the receiver to be made to looser tolerances. The barrel was cold swaged to save time and cost. The weapon was designed to be striker fired, with a fixed firing pin inside the bolt.

When the M3 first entered service, no replacement parts were supplied. The weapon was originally intended to be a low-cost tool that could be discarded when it ceased working. However, due to a temporary shortage of M3s in the middle of 1944, the US Army Ordnance workshops manufactured some spare parts to keep existing weapons operational. There were also a number of issues with the M3 and several improvements were incorporated into the M3A1 model, which went into service in December 1944. During the Korean war, existing M3 guns were converted to the M3A1 standard.

While it was hoped that this gun would be produced in large numbers to replace the Thompson submachine gun in US service, this did not happen and there were about three times as many Tommy guns in service than M3/M3A1 at the end of World War II.

Due to its cheap cost and portability of the weapon, it was issued to paratroopers, tank crews and truck drivers. Even though it was withdrawn from frontline service in the US military in 1957, they were still issued to tank and truck drivers and were carried by US tanks during the 1991 Gulf war. The M3A1s were finally replaced starting in 1992, with the M4 carbine. That means it served in the US military for 50 years (1942 - 1992). It is still used by military forces in other parts of the world.

Actions: Blowback Action - I: Straight Blowback

The blowback action is one of the more common actions used in modern firearms today. It is mostly seen in smaller caliber automatic and semi-automatic weapons such as the Uzi, Sten gun, Ingram MAC-10, Walther PP series (the Walther PPK is usually what fictional spy James Bond usually uses), Makarov etc. Actually there are many different systems that utilize variants of the blowback principle, so the next few posts will be devoted to studying these many variants.

The main principle behind a blowback action is that, at the moment of firing, there is a block of metal called a "bolt" that holds the cartridge in place. This bolt is not locked down, but is held in place by spring pressure. This is a very important point and is the main feature of a blowback action. When the cartridge is fired, the bullet flies out via the barrel and the expanding gases also push against the cartridge case which is still in the barrel. The cartridge case acts somewhat like a piston and in turn pushes on the bolt, which then moves backwards and recocks the weapon. At about the same time, an opening on the top or the side ejects the old spent cartridge case.

The following diagrams will make the concept clear.

In the above figure, we have the bolt in blue. The bolt handle A is used to pull the bolt backwards initially against spring pressure from recoil spring B. This moves the whole bolt (the blue piece) backwards. C is a cavity in the bolt, where hammer E (in rose color) can move). D (in red) is the firing pin. When the gun is initially loaded, a magazine containing cartridges is first inserted into the weapon. Then, handle A is used to pull the bolt back as shown in the figure above. As a result of this, the hammer E rotates about its axis and is cocked. At the same time, due to the pressure of magazine spring F, a new cartridge is pushed upwards into the receiver. When the bolt handle A is released, the front face of the bolt (the blue part) pushes the cartridge forward so it is pushed into the chamber in front of barrel H. The hammer E still remains in its near horizontal position because it is held in place by the trigger bar (which is orange) connected to trigger G.

Now the user pulls the trigger and the following happens:

The user pulls the trigger and the trigger bar releases the hammer E, which rotates about its axis inside cavity C and strikes the firing pin D hard. The other end of the firing pin strikes the base of the centerfire cartridge, which detonates its primer and then ignites the main propellant material in the cartridge. The expanding gases drive the bullet through the barrel H and also push back on the empty cartridge case left behind in the chamber, which in turn pushes back on the bolt.

The entire bolt assembly (the blue part) now moves backwards due to the pressure from the gases acting on the (now-empty) cartridge case. As a result of the bolt moving backwards, the hammer E is now rotated back down until the orange trigger bar holds it down Meanwhile, the opening on top of the receiver is now open and the old cartridge case is pushed out through here by the action of magazine spring F. The bolt moves backwards and compresses the recoil spring B. When the bolt has moved backwards to its utmost, the recoil spring B then expands and pushes it forward into place. While moving forward, the bolt picks up the new cartridge and pushes it into the chamber and the gun is now ready to fire again.

Obviously, for best results, the bolt should not start to move backwards before the bullet has left the barrel and the pressure in the chamber has dropped to a safe level (you don't want a lot of high pressure gas to flow back into the magazine area and out the ejection port and blow out everything in between). In a straight blowback action, this is achieved by making the bolt much heavier than the bullet mass. Therefore when the cartridge is first fired, the bullet shoots out the barrel, but the force acting on the empty case doesn't push the bolt back instantly due to its inertia. By the time the bolt starts to move backwards, the bullet has left the barrel already and pressure of the gas in the chamber has already dropped to a safe level and the bolt and empty cartridge case are moving back on pure momentum alone. Given the larger mass of the bolt and the stiff recoil springs, blowback weapons are usually a bit harder to cock initially. That is why, for most practical purposes, blowback weapons generally tend to use smaller and lower powered ammunition.

The following video shows a MAC-10 submachine gun, which uses a straight blowback action.

And now, a girl firing a Walther PPK pistol. Notice in the beginning of the video about 10 seconds in, when she loads the pistol and pulls the slide back the first time to cock it.

As we alluded to above, in a straight blowback action, the bolt assembly has to be heavy so that it doesn't move back the instant the cartridge fires. As a result of this, straight blowback weapons tend to be heavier than other similar sized counterparts that use different mechanisms. Some manufacturers, such as Hi-point Firearms, try to get around this by making the other parts of the weapon out of lightweight polymers. While using plastics does reduce the weight of the firearm a bit, they are still heavier than, say, recoil action firearms. On the other hand, the mechanism is very simple and easy to manufacture than a recoil action.

In the next few posts, we will study other blowback actions that use different ways of delaying the bolt assembly from moving backwards when the cartridge ignites.