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The Iconic Sten MK III Submachine Gun of World War II

While the Sten MK II is typically considered simple and cheap, the later MK III model was even simpler and less expensive to manufacture.

The Iconic Sten MK III Submachine Gun of World War II

The Sten Gun series is known for being simple and cheap to make, but the most economical of the series was the Sten MK III seen here. (Photo courtesy of Ruben Mendiola and DealerNFA.com)

There are a number of submachine guns famous for their service during World War II. These include the American .45 ACP Thompson series, the Soviet 7.62x25mm PPSh-41 “papasha”, and the German 9mm MP 40. Then there is the British Sten Gun. The Sten is best known for being extremely cheap to make and almost crude in its construction, especially compared to classically-manufactured designs like the Thompson or MP 28. It’s simple and inexpensive nature, requiring few complex machining operations, was a valuable virtue in the dark days following Dunkirk and the Fall of France in 1940.

The Sten Gun family is known for being built…cheap. The simple tube construction of the receiver, crude welds, basic firing mechanism, agriculturally simple stock…everything was done with an eye towards economy and mass production. Usually when thinking of the bargain basement Sten Gun, one thinks of the MK II model. While the MK II was indeed very economical to build, costing only about $11 to manufacture compared to a Thompson which cost $200, it was not the “cheapest” of the Sten Guns. That title goes to the even simpler Sten MK III.

Before we dive into the “cheapest of the cheap”, let’s take a quick look at the basic design. First off, the name “Sten” is actually an acronym. Properly written it is STEN, which stands for the names of its two designers and the facility it was designed at. The weapon’s chief designers were Major Reginald V. Shepherd and Harold J. Turpin, hence the “ST”. The “EN” is from the first two letters of the Enfield factory.

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England entered World War II without a submachine gun and so purchased American Thompsons, but these were extremely expensive at $200 per gun. (Photo courtesy of Ruben Mendiola and DealerNFA.com)

While the design was simplified from the fancier MK I model, the heart of the Sten never changed. Chambered for 9x19mm Parabellum, it operated via simple blow-back and fired from an open bolt. Layout of the basic design was similar to the British Royal Navy’s Lanchester submachine gun, itself a copy of the German MP 28. However, the designs and their construction were very different. The Sten has a simple bolt with a fixed firing pin and extractor which rides inside a tube receiver in front of a recoil spring. A fixed ejector is welded to the left side of the receiver just behind the magazine port. A 32-round magazine locks into place horizontally on the left side of the gun, giving it its distinctive appearance. This makes it easier for a soldier to fire from a low prone position. The magazine is a dual-column design with single-feed and was the weak point of the entire design.

On the MK II model, a 7.75-inch-long barrel was held in place by a short handguard which threaded onto the front of the receiver and then was locked into place by a spring-loaded pin on the mag well assembly. When not in use, the mag well could be rotated 90 degrees placing the opening for the magazine at 6 o’clock. Rotating it in this fashion would cover both the feed and ejection ports to keep dirt and debris out of the action. What will surprise many is that the Sten is selective-fire. A cross-bolt forward of the trigger allowed the gunner to select Semi-Automatic or Full Automatic fire. The basic Sten Mk II was easy to produce with few complex machining operations required. Many parts were stamped and spot welded together. During the dark days of the war, England needed submachine guns and a lot of them, and the Sten was designed for simple mass production.

As the British mobilized for war, many companies which had never manufactured weapons began contributing to the war effort. One of these was a toy and bicycle manufacturer in South-West London called Lines Brothers Ltd. The company name comes from the fact the firm was founded by three brothers, Walter, Arthur and William Lines. They sold toys under the trade name Tri-ang (the meaning of which is three lines form a triangle) which were produced in a very large and modern facility in the Merton borough of London. They were particularly adept at producing items from pressed metal, and at one time claimed to be the largest toy manufacturer in the world.

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During World War II, British troops faced Germans equipped with 9mm MP 38 and MP 40 submachine guns. (Photo courtesy of Ruben Mendiola and DealerNFA.com)

Walter Lines, having examined the MK I and II Sten, realized that while he had no experience building small arms, he and his team were very experienced when it came to designing and manufacturing things from stamped sheet metal. So, in 1941, Walter designed a new model of the Sten Gun, specifically for ease of mass production in his facility, with his existing tooling and equipment.

Whereas the Sten MK II was built using a short tubular receiver with major pieces, such as the barrel being easily removed, the new design was quite different. The foundation for the new model was a piece of flat sheet metal pressed into a long tube with the joint on top. This pronounced joint was welded together forming a rib, and the resulting long tube became both the weapon’s receiver and barrel shroud/handguard. Rather than having a rotating mag well and removable handguard and barrel these items were simplified and permanently fixed in place.

While the MK II was built using 69 parts, Walter Lines was able to reduce this number to just 48. Expensive machined parts were limited to the bolt and barrel. Simple stamped metal parts were riveted or welded together. Production time for a complete submachine gun was reduced to just 5.5 hours. The end result operated the same as the Sten MK II, fired the same ammunition, and fed from the same magazine. It was lighter than the MK II though, but it did not share complete parts commonality with the MK II.

Prototypes were tested by the War Department during the Winter of 1941/42. While some improvements needed to be made, the tests were successful, and the Lines Brothers received a contract to produce 500,000 of the new model. Designed for economical mass production the new MK III was the “cheapest” of the Stens.

The Sten MK III is simple to operate and effective at typical submachine gun distances. The standard Sten 32-round magazine is very hard to load cartridges into once you get past a few rounds, and you will need a magazine loading tool. Again, the magazine is the weak point of the entire design, keep them clean and inspect the feed lips.

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Remember, this is an open bolt design with a fixed firing pin. Bolt forward on a loaded magazine is not safe. If the bolt is forward on a loaded magazine it is possible for a jolt to cause the bolt to slide back in the receiver far enough to strip a round from the magazine, but not far enough to be caught by the sear. If this happens, the recoil spring will drive the bolt forward and, due to the fixed firing pin, it will fire a single-shot. This can happen simply by jumping out of the back of a troop transport, which led to injuries and death throughout the Sten’s wartime service.

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The British found the solution to their submachine gun dilemma in the Sten Gun series, with the MK II seen here. (Photo courtesy of Ruben Mendiola and DealerNFA.com)

To operate the gun, first retract the bolt handle on the right side of the tube receiver. Pull it back and up into the safety cut-out notch. Insert a loaded magazine into the magazine well until it locks into place. The cross-bolt selector is marked with “R” on the left side and “A” on the right. Simply pushing the “R” (so the cross bolt moves from the left to the right) provides semi-automatic fire. Pressing on the “A” (moving the cross bolt from the right to the left) provides full automatic.

The front sight is an unprotected blade while the rear sight is a fixed aperture set for 100 yards. Sight picture is good, and the aperture is large enough for effective use in low-light conditions. No issues here. Pull the charging handle back slightly and rotate it down into the main bolt handle slot and the weapon is ready to fire. With the Sten set on “R” pulling the trigger will cause the sear to release the bolt and the recoil spring will drive it forward, strip a round from the magazine and feed it into the barrel, the fixed firing pin will fire the cartridge. After the projectile exits the barrel and pressure has dropped to a safe level, the bolt will be forced to the rear extracting and ejecting the spent case. When the bolt has reached its rear most travel the recoil spring will push it forward to be caught by the sear.

With the selector on “A”, the cycle is the same except the sear will not catch the bolt until the trigger is released. So, the gun will continue to fire until it either runs out of ammunition or the trigger is released allowing the sear to rise up and catch the bolt. When firing on semi-automatic, the Sten Gun has both mild felt recoil and a mild report. It does have an odd firing sensation due to it being an open bolt design. When you press the trigger there is a noticeable delay as the bolt suddenly clunks forward loading a cartridge and then firing it, and then after it cycles there is the sensation of the bolt being caught by the sear.

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The Sten MK III was a simplified and cheaper alternative to the MK II and was produced in large numbers during the war. (Photo courtesy of Royal Armouries)

In the hands, the Sten MK III submachine gun is short at 30 inches and fairly light at seven pounds. The charging handle is easy to reach and manipulate, the selector operates with a distinct “click” and the sight picture is good. Magazines insert easily and lock into place with a click. Locking a fully loaded magazine into the gun does unbalance it a bit, due to it jutting out of the left side of the gun. The buttstock (both the “T”-style and the “Loop”) is very simple, has a short length of pull and while not the most comfortable thing in the world, it works.

The safety system is crude and slow, having to withdraw the bolt handle from the safety notch, but no worse than the German MP 40. Cyclic rate of fire is about 500 to 600 rounds per minute. British textbook firing technique is to grasp the front of the receiver tube with your left hand, not the magazine. The MK III has a short handstop directly in front of the ejection port to prevent your left hand from sliding back over it. It was permitted to grasp the mag well, but grasping the magazine itself can cause problems with feeding.

The design is fairly easy to control on full-automatic. Triggering off short bursts, or even single rounds is not difficult. The gun merely vibrates around a bit firing full auto. The Sten gun added much needed short-range firepower to the British infantry squad, and nicely complemented their bolt-action Lee Enfield service rifles. They were particularly effective clearing houses, bunkers, and defensive works, fighting in the woods and assaulting enemy positions. They were also useful for fending off assaults at short range. The Sten could quickly and efficiently put multiple rounds onto multiple targets, by firing it in short bursts. Due to it being chambered for 9mm pistol ammunition, its effective range was just a bit over 100 meters.

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A British radioman is seen here equipped with a 9mm Sten MK III submachine gun. At 30 inches long, and weighing seven pounds, it was compact, light and effective.

Initially, British 9mm ball service ammunition drove a 115-grain FMJ with a gilding metal jack and a lead core at 1,100 to 1,150 fps. Designated Cartridge S.A. Ball 9mm Mark IZ, it was approved for service in December 1941 and was intended for use in Lanchester and Sten submachine guns. 9mm ammunition was also sourced from the United States and Bolivia, and large quantities of captured ammunition were put into service. The Mark IZ load did not prove ideal for use in submachine guns though and in Sept 1943 a new higher velocity load was approved. Designated Cartridge S.A. Ball 9mm Mark IIZ, it initially had a muzzle velocity of 1,250 fps but this was increased to 1,300 fps in 1944. This higher velocity 1,300 fps load increased the Sten Guns reliability.

Field stripping the Sten MK III is very easy. The stock removes easily without tools allowing the recoil spring and bolt to be slid out. The firing mechanism is very simple. It is not a hard piece to maintain, but the charging handle slot and ejection port are large entry points for dirt, dust and debris which can cause issues.

Yes, the Sten MK III is a bit crude. Fit and finish will not impress you. The gun is made up of cheap stamped steel parts spot welded together with a cross-pin keeping the firing mechanism together. Even so, it proved an effective weapon at the short distances submachine guns excel at. The MK III was the second most produced model of the Sten series, just behind the MK II. Lines Brothers Merton Works facility reached a production rate of 70,000 guns a month and produced a total of 876,886 of them.

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The Sten series would be officially replaced by the Sterling submachine gun in 1953, but would soldier on into the 1960s. (Photo courtesy of Ruben Mendiola and DealerNFA.com)

In use, the MK III did not prove as reliable as the MK II. Since the barrel and mag well were non-removable, if they were damaged the weapon had to be scrapped. In the fall of 1943, it was decided to end production of the MK III and concentrate on the MK II. The MK IIIs went on to see heavy use, they were dropped behind enemy lines to partisans and issued to the British Home Guard. The Sten series soldiered on after World War II and saw service again in Korea before being officially replaced by the Sterling submachine gun in 1953. It remained in British service into the 1960s before finally being withdrawn.


About the Author

David M. Fortier has been covering firearms, ammunition and optics since 1998. He is a recipient of the Carl Zeiss Outdoor Writer of the Year award and his writing has been recognized by the Civil Rights organization JPFO. In 2007, he covered the war in Iraq as an embedded journalist. He has written extensively on opposing forces small arms, ammunition and optics and has traveled through Russia, Europe, the Middle East, and South America. His writing has been translated into both Russian and Mandarin. He was a regular on the Outdoor Sportsman Group’s network television from 2003 to 2020. He is currently the Editor of the OSG Prepping title , and the Executive Editor of Firearms News. Prior to 1998, he was in the Aerospace and Defense industry.


If you have any thoughts or comments on this article, we’d love to hear them. Email us at FirearmsNews@Outdoorsg.com.




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