A True British Classic: The Sten Gun

A True British Classic: The Sten Gun

Introduction:
Back at the 2009 Shot Show, Century Arms introduced their Sterling Sporter 9mm carbine. Some overseas military had surplused thousands of British manufactured Sterling MK IV SMGs and Century purchased them; cutting them up into parts kits. They then sent the kits to Wiselite to be reassembled into legal semi-auto only carbines for the US civilian market. I saw one of these Sterling Sporters in early 2009 and broke one of my long standing rules: no open bolt to closed bolt conversion guns in my safe. I just liked the feel and looks of the Sterling too much not to have one. Actually, i purchased one for myself as a wedding gift in August of that year.
It was a great carbine and I did a little thread on it. In that thread i took some cheap shots at the older British Sten gun, to highlight many of the Sterling's improvements. Forums member Sten Freak quickly put me in my place by pointing out many of the Sten's admirable trates. So a short time later i decided maybe having a semi-auto Sten would also be neat.
I soon discovered there was a problem with this idea however, as no US company had really ever mass produced a closed bolt semi Sten. So finding a good reliable one wasn't easy. Eventually in 2011, i found an old Catco SA2, which is a legal semi Sten MK II clone. Catco had a mixed reputation and has long since been out of business. Still it was really the only company that even came close to semi Sten mass production; they themselves made a few hundred carbines and pistols back in the late 1990s and early 2000s.
I bought the SA2 and was quite content with it. I thought that would be it, and it was for about two years. It ran well and really looked the part of a british WWII SMG. It went well with my Sterling Sporter too.
Then earlier this year, i found a good deal on a pair of preban British made Sterlings and couldn't pass them up. I did a thread on them already, which i will link to below for anyone who missed it and would like to read it. After upgrading my Sterling, i thought it might be worth looking into doing the same with my Sten. Now, there is no such thing as a preban semi-auto British made Sten, so i was thinking it might be nice to buy a parts kit and have another model built up. Also, while the Catco was nice, it was not as authentic as i could have wished. There were some things that could be done to make a legal semi Sten closer to an original.
Long story short, through a friend I met a great gentleman down in Florida who agreed to build me both a Sten MK II and MK V to my specifications, from parts kits. He supplied the MK II kit and I had the MK V. Even better, my friend sold me his MK III, which was also built by the guy in FL. So now I have a nice set of 3 semi Stens, all done by the same guy with the same high quality conversions. In each case, he tried to keep them as original as possible, while sticking to the ATF's rules.
While doing these builds with him, i collected a goodly bit of information about the Sten machine carbine from WWII. Since I had already done a thread on the Sterling, one for the Sten only seemed natural.
Sten Freak has once again helped me a lot with information and photographs. In addition, just recently, Apex has started offering all 3 major Sten variants as parts kits. So there is renewed interest out there in semi-Stens, with kits on the market again.
So with all of that said, lets dig into the firearm which became nicknamed as the "Plummer's Abortion!"

Background & Development:
The now legendary Sten machine carbine was one of those developments that was a long time in coming. The British expressed little interest in the then new Submachinegun class of smallarm, which was first seen during the last year of World War I. Development of the SMG continued during the Interwar era, with the famous Thompson entering into production in the early 1920s. Germany, Russia, and Finland all developed their own during the 1930s also. Britan however, still lagged behind and was not fielding an SMG when war broke out in 1939.
At the Battle of Dunkirk, the British Army lost a large number of firearms and ammunition. While a good part of the men themselves were successfully evacuated, much of their equipment had to be left behind. As a result, the British had a sovear shortage of firearms to arm themselves with, when the Battle of Britan began.
With the very real possibility of invasion of the British Home Islands by Nazi Germany, the nation's leadership called for a new automatic weapon, which could be produced quickly and cheaply, could be made from existing materials, and something that soldiers could easily be trained to use effectively in combat. In the summer of 1940, the call was put out that the British Army was in need of a Submachinegun.
The first design to appear was the Lanchester, which was essentually a British copy of the German MP28. The Lanchester even fed from MP28 magazines and used very similar parts. In August of 1940, 50,000 such weapons were ordered from the Sterling Arms Co. They were meant for the RAF, as airfield guard carbines, but by the time production was in full swing, the RAF had found another SMG to fill the role. So instead, most Lanchesters were given to the Royal Navy or to British Allies. At the same time, the British were buying up as many Thompson M1928s as Auto-Ordnance could supply. However, the SMG was in an uncommon caliber for Europe and not nearly enough could be made available. The Thompson was quite costly and time consuming to produce.
The Lanchester was a solid reliable gun, but was somewhat complex to manufacture; costing nearly two dozen man hours to make and assemble the parts. It was good, but Britan was looking for something better suited to its hour of need.
Even as production for the Lanchester was getting underway in late 1940, ROF Enfield was ordered to design an easily mass produced SMG, which fired a common caliber. This of course would become the Sten, with the first prototype tested a mear few weeks later in January of 1941.
The name 'Sten" is frequently explained as an acronem for the weapon's designers. S for Major Reginald V. Shepherd, T for Harold J. Turpin, and EN for ROF Enfield. Though a few sources claim the EN stands for England; maybe both could even be true? The Sten's design was loosely based on the MP28 and Lanchester, and also shared some features with the MP38. It was made from many stamped metal parts, as well as off-the-shelf piping and screws. In fact, really only two components, the barrel and bolt, actually needed to be machined. Everything else was easily fabricated in a basic machineshop. The Sten utilized spot welded construction and did not require highly skilled labor to complete.
It was officially adopted into British service as the Sten Machine Carbine MK I in the Spring of 1941. Production was quickly ramped up and by the Summer, thousands were already being turned out, with components made in dozens of factories all over England. The new weapon saw its first major action in 1942 during the Dieppe Raid. The Raid itself did not go terribly well for the Allies, however it did proove the Sten to be a viable, if not perfect, combat weapon.

General Sten Info:
The Sten was a classic third generation SMG. It was built from many stamped parts and fired from an open bolt. It could be set to either single shot or fully automatic, by use of a very simple cross-button selector. It had a 7.8" long barrel, with a heavy walled chamber. The bolt was its other solid massive part, being a single heavy piece design with fixed firing pin. It was intended to be milled from steel, however in 1942 and 1943 Britan, steel was in short supply. So some Stens were fitted with bolts made from cast bronze (often mistaken for brass). Surprisingly, these bolts held-up ok in combat, though naturally they had a shorter service life. Somewhat unique, the SMG's magazine well was mounted to the left side of the receiver tube, with the magazine itself sticking out horizontally. Sten magazines were of a double column, single feed type, a design borrowed from the MP28. Their standard capacity was 32 rounds, though older 50 rounders from the Lanchester would also work in the Sten. Later on, some magazines would be modified to hold 20 cartridges in a single column. This was done by some nations such as Israel and India, to improve feeding.
The Sten never had much in the way of safety features. In the beginning, its only real manual safety was a 'J hook' notch cut into the receiver along the cocking handle's track. The bolt could be pulled back and the cocking handle slipped into this notch, physically preventing the bolt from moving forward. This system was at best only partially effective. One major issue was if the bolt were forward and the SMG were bumped hard enough, the bolt could move back far enough to strip a round from the magazine, but not back far enough to engage the sear. The result would be a run-away slam firing, thanks to the fixed firing pin on the bolt face. After a few years, a new style of cocking handle was introduced. When the bolt was forward, this new handle could be pressed inward. Doing this would push the far end of the handle into a notch in the receiver, thus locking the bolt in the forward position. Many older Stens would be retrofitted with the improved safety system. Still yet, the firearm never had a true manual safety, such as on the Thompson or Suomi.
The Sten was designed from the beginning for mass production, and to require as few resources as possible. The only two parts that actually had to be milled, and thus required skilled labor, were the barrel and bolt. Really all of the other parts could be made from stampings and put together by workers with only minimal training. The SMG's receiver tube and stock strut were made from tubing with the same diameters as off-the-shelf piping. The screws even used a common thread pitch. Relatively few welds were required to build a Sten, and they did not need to be precise.
At the start of manufacturing, 1 Sten required roughly 12 man hours to complete, which was half the time required to build a Lanchester SMG. However, as time went on, workers gained experience, the design was simplified, and production techniques improved, a Sten could be created in as few as 5 man hours. The SMG had few parts, with the MK III version actually having a total of only 47.
The Sten was produced by several factories, including: Royal Small Arms Factory, Enfield, Singer Soing Machine, BSA, ROF Fazakerley, ROF Maltby, ROF Theale, Berkshire, Lines Brothers Ltd, and Long Branch Canada. Additionally, at least a dozen smaller firms were subcontracted with to make individual components, which were shipped to the various main facilities for use in final assembly. The vast majority of Stens were built during World War II, between 1941 and 1945. No one knows exactly how many were made, but estimates range from 3.6 to 4.7 million in total, counting all variants.

The MK I & MK I*:

The first Sten variant was manufactured in 1941 as the MK I. It was to be sure an economy weapon, but it did have a few refinements, which would be removed on later models. For example it had its barrel fully enclosed by a shroud and a basic muzzle brake at the end. It had a rifle shaped metal skeletonized stock, similar to the later 'loop' stock but more ornate. This stock had an attachment point for an Enfield rifle type sling. It also had a foregrip made of wood or metal. About 100,000 of the MK I variant were built, before being replaced by the MK I*.
The MK I* featured the first of many cost savings changes to the Sten design. It lacked the foregrip and brake, and no wood at all was used during its construction. Also, the rifle sling mount was removed from the stock. About 200,000 MK I*s were built during the latter half of 1941, most by Singer Soing Machine in England. These models were intended primarily for use by ground forces, as they had barrels rivited into the receiver tube, and fixed side feeding magazine wells. Only the stock was easily removable. The metal parts received an inexpensive, but decent enough chemical bluing treatment.

The MK II:

Shortly after the adoption of the MK I Sten, testing began on a second model, intended for use by airborn and mechanized forces. It was officially adopted as the MK II at the end of the Summer of 1941. The MK II had further simplifications from the MK I* model, which made it both faster and cheaper to mass produce. It used even more stamped parts, required less fitting, replaced some rivits with welds, and had very basic furniture. To make it easier for transportation and storage, it featured a new style of magazine well, which could rotate down 90 degrees when not in use. It also had a barrel which could quickly and easily be removed from the receiver in the field. As with all Stens, the stock was very easy to detach also. These features made the MK II perfect for paratroopers and easier to store in military vehicles.
The MK II had a shortened barrel shroud, which left several inches of the barrel exposed. There was just barely enough room for a hand in fact, it was so short. It had very basic fixed iron sights, with a blade front and aperture rear. The cocking handle was little more than a metal rod and there was no pistol grip at all in either the front or back. Two types of stock were primarily used with the MK II. The 'loop' stock as its known today was a simplified version of the original skeletonized stock. The 't' stock was even more crude, with a single piece of pipe surving as a strut, welded to a buttplate. The loop stock was more comfortable and attractive, but the t stock prooved to be much more durable, so it became the accepted standard in the British military.
The first contract to build MK IIs was given to Long Branch in Canada in late 1941. By the beginning of the following year though, several factories in Britan were also turning out the new variant, including: Enfield, Fazakerley, Theale, and BSA. No one knows for sure exactly how many were produced during the war, but estimates range from 2,000,000 to 2,600,000, including all factories. Regardless of the actual number, the fact is that the MK II was by far the most mass produced version of the Sten gun. It saw widespread combat in the European Theatre throughout the war. Many were also given to British Allies and underground resistance movements fighting the Nazi Germans.

The MK III:

The MK III variant returned to the first generation layout of the Sten. It was more like the MK I* than the MK II in otherwords. Its receiver tube, barrel shroud, and magazine well were assembled together as a single unit, and couldn't be separated without a torch. The barrel too was rivited into the receiver and was covered mostly by the shroud. The MK III was the simplest and cheapest Sten version ever fielded. It used parts made mostly from stampings, including a receiver which was made as a flat and then rolled up and welded into a tube. Some SMGs even had a stamped sear, rather than one made from a milled piece of steel, as was common on earlier Stens. Most parts were spot welded together, with few rivits used. Rather than being held on with screws, the dustcover was dimple pressed into place. The most common stock found on the MK III was the 't' version. The MK III did have a few advantages over the MK II, so it wasn't just an economy model. Its fixed magazine well did give it improved reliability, and its longer barrel shroud was easier to use as a handguard. It had an actual sling swivel on the underside (later some SMGs would also have a second swivel added to their buttstocks), and a small metal flap added in front of the ejection port. This flap was meant to insure the shooter's hand did not move too far back on the tube. Finally, some found the ridge ontop of the receiver, which was a side product of the manufacturing process, made the weapon easier to sight along and aim down.
Production of the MK III began around April of 1942, with Lines Brothers, a toy manufacturer experienced in working with stamped metal parts, being the biggest producer. IN all, approximately 900,000 SMGs of this type were made during a 2 year run. As evidenced by its configuration, this Sten was mostly meant for use by ground forces, like the MK I before it. It was popular among military base and factory guards on patrol.

The MK V:

After the tide of the war started to turn in favour of the British and their allies, the need to economize in firearms production lessened. As a result, after the very simplestic MK II, the next Sten would move back towards a slightly more ornate design. The MK V featured a return to wooden furniture, including both a buttstock and finger grooved pistol grip. The stock mounted like any other Sten stock, but the introduction of a true pistol grip meant that the lower trigger housing and dustcover had to be slightly redesigned, with the trigger itself moved forward. The buttplate was made of brass and even had a trapdoor to store a small cleaning kit. The new model had an Enfield No. 4 type front sightlocated on the end of the barrel. This sight was drift adjustable for windage and had protective ears. A No. 4 bayonet could also be mounted onto the MK V's muzzle. The cocking piece was enlarged, making it easier and more comfortable to use. Early models also came with a small wooden foregrip, however it was found to easily break and was thus removed from the design.
Generally speaking, this model of Sten had better fit and finish than previous ones. It also introduced a new method of treating the external metal parts, which Britan would carry over to the Enfield and continue to use on the L1A1 SLR. Where as earlier Stens were finished with a chemical bluing process, the MK V featured a phosphate coating topped by a black paint substance, which was more rust resistant. For all of its unique appearance though, the final wartime Sten was simply a dressed up MK II. It had the same removable barrel, short shroud, rotating magazine well, and fixed aperture rear sight. It was no more reliable, accurate, or safe to use than any other Sten. It did look more like an American Thompson, which the higherups thought would inspire more confidence with the troops. This really didn't proove to be the case in the end. Soldiers didn't much care for the additional weight that the MK V carried, thanks to its wooden furniture and more complicated barrel assembly. So surprise, they cared more about a weapon's usefulness in the field, than its superficial appearance.
About 525,000 MK Vs were built, with production beginning in February 1944, and lasting until shortly after the end of the war. The two main factories responsible were Theale and Fazakerley. MK V production did replace that of the MK III, however MK IIs continued to be made in Canada and elsewhere. The MK V did see combat, mostly in Europe following D-Day. It was most famously used in Operation Market Garden at Arnhem, by British paratroopers.

Other Stens:
As you noticed, there is no MK IV listed above. This is because this model never went into production. It was a compact version, basically a 'machine-pistol' with a 4" barrel, which was intended for paratroopers.
The version of the Sten manufactured by Long Branch Canada was similar to the British MK II, but different enough that it was officially designated as CMK II. The differences between the two versions were minor, relating mostly to slightly different manufacturing techniques. Also, the CMK II was normally built with the skeletonized loop stock.


There was also a MK II(s), which was a dedicated suppressed version of the standard MK II. It had a perminant suppressor device, which was quite effective at reducing audible noise. The trade off was that the MK II(s) had a shorter effective range and was meant to be fired as a semi-automatic only. Full-auto was reserved for 'emergency use only' as using it would quickly wearout the suppressor. There is no hard evidence to support the claim that bronze bolts were most often used in suppressed Stens, as they were quieter than ones made of steel.


A silenced version of the MK III was designated as the MK III(s), and the MK VI, breaking with tradition, was the name given to the silenced version of the MK V.
Interestingly, in 1944 in Germany, DWM/Mauser built around 28,000 near-exact clones of the Sten MK II. These clones were correct right down to the fake British proof markings and were nicknamed Gerat Potsdam guns. No one knows for sure the purpose of these weapons but use your imagination.



During the final year of the war, when Germany's factories were starved for resources, Mauser once again turned to the Sten design, this time to equip Germans to fight against the Allies. The MP.3008 Volks Machinenpistole was a near exact copy of the Sten, with the biggest difference being its vertically oriented magazine. It was produced with copies of both the loop and t stocks, and a few examples have even been discovered with basic stocks made of wood. What made the SMG attractive to Britan in 1941, made it so for Germany in 1945. In all, about 10,000 were built, some made just days before the end of the war.
Thanks to its simple construction, many underground resistance movements in Europe during WWII were also able to copy the Sten, and build their own guns nearly from scratch. Again, the only two parts that really needed to be built to withstand the stresses of firing were the barrel and bolt. The other parts could be built from a wide range (and quality) of materials. Both Norway and Poland were known to have built such guns.
Also during the 1940s, Stens were cobbled together from whatever could be found by Jews in what was then Palestine. These brave souls with home made SMGs were the backbone of the movement, which would eventually result in the formation of the modern state of Israel in 1948.
The Sten had a great influence on other SMGs designed during WWII too. For example its barrel and bolt system were borrowed by Carl Gustav when it was developing what would become the M/45, aka the Swedish K. Its simple stamped steel construction would inspire Soviet engineers when developing the PPS43, during the fight for Leningrad. The Sten would have an equally strong influence when the Americans wished to replace their expensive and complex M1 Thompson, with something better suited to wartime mass production. Thus the M3 Grease Gun has many aspects remenicent of the Sten, such as its receiver tube construction, sights, and magazine.

After The War:
After the war, all 3 main versions of the Sten served together, side by side in Common Wealth units around the world. The Sten was the standard issue SMG in Britan, Canada, South Africa, and India. It was also in widespread use in several other nations, including Australia and New Zealand. So many were made during the war, and the design was so easy to repair and keep running, that there were more than enough Stens to go around without the need to build replacements.
This is why in part the British military was so slow in replacing the Sten with the Sterling SMG. The Patchett MK I, the original model of the Sterling, first appeared in 1944 and was even ready for field use by 1945. It was tested extensively and prooved itself superior to the Sten and other SMGs but was not officially adopted into service until 1951. Even then, it wasn't until 1956, that it went into widespread use. At the same time, after adopting a new designation system for military equipment, the British military renamed its various Sten models. The MK II became the L50, the MK III the L51, and the MK V the L52. The Sten was slowly phased out in favour of the Sterling during the late 1950s and early 1960s, but it was a gradual process. It seems clear that the military wasn't overly concerned with replacing the SMG which saw Britan through one of its darkest hours. It hung on in Canada too, not being replaced by the C1 Sterling variant until the mid to late 1960s. The young Israeli Defense Force used it during the 1950s, before retiring it in favour of their own Uzi SMG. The Sten went to Vietnam in the 1960s in the hands of Australian, New Zealand, and even American soldiers. It continued to see heavy use in Africa during the Cold War era, and unlicensed copies were built in India, where it remained the standard SMG until at least the 1970s. Finally, the Sten design had a strong influence on various postwar SMG models created all around the world, such as the American Ingram MAC-10 and S&W M76, German Walther MPL, Argentine FMK-3, Belgian Vigneron M2, Bulgarian Arsenal Shipka (named after a brand of cigarette by the way), Czech Vz.23 series, and numerous others.

A Very Brief Look At The Sterling:



The Sterling SMG was the brainchild of George W. Patchett, who was the chief designer at the Sterling Armaments Co. during WWII. As early as 1943, the British General Staff began to look into an improved model of SMG and the following year, the Patchett MK I was ready for field trials. However, the end of the war put an end to massive defense spending in Britan, so for the rest of the decade the Patchett was tinkered with and put into various trials.
That said, by the early 1950s, the British realised their standard issue smallarms were horribly out of date. Actually, the Sten gun was one of the more modern firearms their soldiers took with them to fight in Korea. Otherwise, they carried the Lee-Enfield No. 4 (a bolt action not dissimilar to the blackpowder Lee-Metford from the 1880s), the Enfield No. 2 MK I* (a breaktop revolver based on the Webley, also first fielded in the 1880s), and the Bren MK I and II (a pre-war LMG, which while reliable and well made, had its own shortcomings with its roots in WWI). So a crash program was begun to update British equipment.
The Sterling was one of the first new firearms to be adopted in 1951 as the L2A1, and thanks to several years of refinement, it was basically good to go right away. The L1A1 SLR, an FN FAL variant, replaced the old Lee-Enfield in 1957; and while the Bren continued in service, it received a facelift to become the L4A1.
Looking at the Sterling is really informative if one wants to discover what the British felt were the critical shortcomings of the Sten. Like the Sten, the Sterling used the 9mm Parabellum cartridge and fed from a horizontally oriented magazine. It had a 7.8" barrel and fired from an open bolt. There were major differences though.
The Sterling had a folding metal buttstock, along with a pistol grip made of bakelite. Its magazine well was fixed and its sights were adjustable for both windage and elevation. Both sights also had protective ears. The Sterling's most important improvement was its magazine. It was curved and held 34 rounds. It was of the double column, double feed pattern. Somewhat unique, its follower had 2 rollers, which gave it excellent reliability while also making loading faster and easier. Interestingly, the Sterling's magazine well was reverse compatable, meaning that it could feed from older Sten and Lanchester magazines if the need ever arose. However, the opposite was not true. Sten guns could not use the newer magazines from the Sterling.
The L2A3 version of the Sterling first appeared in 1956, and would ultimately remain in British service for half a century. It was finally replaced by the L85A1 in the 1990s. In a sense, the Sten gun lived on in the Sterling, since the latter was so heavily influenced by the former.

Sten Stats:
Weight: 3.2 kg (7.1 lb) (Mk. II)
Length: 760 mm (30 in)
Barrel: 196 mm (7.8 in)
RPM: 500
Range: sights set to 100 meteres
Magazine Capacity: 32 rounds Standard, 20 & 50 rounds in special cases

the Culture Of the Sten:
The soldiers who carried the Sten gun often developed a true love/hate relationship with it. It was rather ugly to many and not especially ergonomic. It wasn't terribly accurate and had crude fixed sights. Its magazines were really its weakest point. They were difficult to load fully without the use of a tool, and even if one managed to do it, they often were unreliable with more than 28 cartridges in them. Then again, they were just plain unreliable thanks to the double column, single feed design. This design also made them sensitive to dust, dirt, and mud. This is why some militaries converted Sten mags over to single stack 20 rounders. Probably the biggest complaint though was that the gun wasn't all that safe. Like many open bolt SMGs, the Sten could fire accidentally if dropped on its butt or for that matter, if dropped at all. The cocking piece safety lock was a late addition to the design, though older guns were usually retrofitted to have it. Otherwise, the Sten only had the bolt notch as its safety.
On the otherhand, the Sten was reasonably reliable and accurate when kept in good working order and cleaned regularly. It was easy to learn to use and to train others to use. Field stripping was also easy, along with takedown on the MK II and V models. The gun was easy to store, transport, and even hide. When assembled, it was quite lightweight and compact. Most importantly, it was fast and cheap to produce, so Britan was able to build millions during WWII, meaning its soldiers were given enough firepower to do what needed doing. Few other firearm designs in history have been so effective, at such a low cost. Considering the Sten's counterpart in the British military was the bolt action Lee Enfield No. 4 MK I, its rapid fire and large capacity were greatly appreciated.
For all these reasons, and more, it was given several nicknames, such as Plummer's Nightmare / Abortion and Stench Gun. There was even a famous poem written about it by a soldier called "Owde To A Sten." Hey, the Sterling never inspired poetry!

Gallery:
Sten MkI , probably museum picture:


Brit soldiers with Sten MkII


German officer examining Sten MkII


Gurkha with knife and part of Sten MKIV:


Israeli woman with Sten MkII:


Sten MkII next to Finnish (larger) Sten MKII:


Sten MKIIs Vietnam display case:


Sten Freak's Sten MKII:


Sten Freak's Sten MKII with short barrel and sling:


Sten Freak's Sten MkII with suppressor shown not attached:


The L34A1, a dedicated suppressed version of the Sterling:


Semi Stens:
As I said in the Intro, there have been few American companies who have produced complete semi-auto only Sten carbines or pistols. CATCo probably made the most and is the most well known today. Wiselite Arms, the same folks who made the semi Sterling Sporter, received ATF approval for a Sten MK III semi, but the carbine never went into production beyond a few prototypes. Valkyrie Arms, of semi M3 Grease Gun fame, has also offered a MK II semi pistol in limited numbers. What is more common is for someone to buy an 80% complete semi receiver tube and try their hand at making their own at home. Several outfits offer these DIY kits, including Indianapolis Ordnance. I myself had my 3 semi Stens custom built by an FFL-07 down in Florida, by the name of MK Gun Mods.
The ATF has several requirements for a civilian legal, semi-auto, Title 1 Sten gun. Obviously, the fire control group has to be modified in such a way that it is incapable of automatic fire and can not easily be modified to be made FA again. Second, the semi receiver tube can not readily accept an original full-auto Sten bolt. Two methods are commonly used to achieve this. In one, the inner diameter of the tube is smaller than the outer diameter of a FA bolt. With the other, a blocking bar is welded inside the tube, again so an unmodified FA bolt can't be inserted.
Since 1981, the ATF has not allowed semi-auto Title 1 firearms to operate from an open bolt. So semi Stens built since then have to be made to fire from a closed bolt. This requires alteration to the bolt itself, cocking handle, return spring, and firing pin. It also ties in with modifications made to the FCG and possibly other parts, depending on the exact method used. Finally, unless the build is to be an SBR, something has to be done about the original 7.8" barrel. It can be used as is, if the stock is omitted and replaced with a pistol grip. This would be a Sten pistol. Alternatively, the original barrel can have an extension added to it, to bring its overall length up to at least 16.1", which would make it a carbine so it could legally have a stock. Of course, rather than extending the original short barrel, one could always simply purchase an American made 16.1" Sten barrel from any one of a number of venders. Either way, a good bit of work goes into making a solid and reliable (and ATF approved) semi-auto SMG.

My old CATCo SA-2:






I bought this carbine off an individual who was trading up to get an NFA Sten gun. I kept it for a couple of years, and then traded it on myself, to get a more authentic MK II clone. For what its worth, it actually ran well for me.

My Sten MK II with loop stock and early production bronze bolt:






This is the MK II i sold the CATCo to get. I had it built with an original bronze bolt, just because i thought it would be interesting. It has a loop stock, complete with an original cleaning rod stored inside. It also has an early cocking handle and screwed on dustcover. I like it better than the CATCo because it has the J hook safety notch. Also, the original selector switch has been converted to act as a manual safety.

My Sten MK III with standard t-stock:






This is my second MK III carbine from MKGM. I had this one built with the J notch safety, which the first was lacking. It also has the second sling swivel located on the buttstock. The cocking handle is a later enlarged style, and the dustcover is dimple pressed on.

My Sten MK V with original wood furniture:






I had this MK V kit laying around for a bit, and it came with the original barrel. I wanted to have it built up right and i think its the best of the bunch. Its about as correct as one can get, while keeping within ATF guidelines. It has the late style mushroom head cocking knob, which was the one that acted as a push safety bolt lock.

MKGM Sten MK III Pistol:




This is a semi MK III pistol built by MKGM, complete with original 7.8" barrel and paratrooper style rear grip in place of a buttstock. I sold a couple of these on Gunbroker back during the Summer.

Valkyrie Arms Sten SAP2 Pistol:




Quite recently, Valkyrie Arms built up a few hundred Sten MK II pistols, using original 7.8" barrels and custom rear grips. Unlike the CATCo and MKGM guns, the Valkyrie are actually hammer fired, using a highly modified AR15 FCG. These guns are built from MK III kits, which are reworked to appear like MK IIs. I sold 3 of these last Spring and Summer to various individuals.

My British mfg Sterling MK 6 Carbine:






Just for good measure and because it is fun to show off, this is my original British made Sterling MK 6 Carbine. It was imported in the early 1980s, and is as close as one can get to a factory semi Sten.

Our youtube look at the semi Stens & Sterlings:
http://www.youtube.com/watch?v=BZJksBr_uyo
(I have a different MK III now then the one seen in the video)

Final Thoughts:
The Sten is packed with history, which one can feel even when handling semi-auto rebuilds. Its one of the most absolute utilitarian firearms ever adopted by any military, and I am even including Russia in that statement. Its fun to fire and play around with. Its even quite 'modular' especially for its day. For example, there are several stocks that one can buy, which just snap right onto virtually any Sten. I've seen folders and ones that adapt the gun to take an Enfield No.4 wood stock. Parts and accessories are inexpensive at the moment, including magazines, slings, and dropcases.
The Sten has little to no modern tactical usefulness, which is part of the reason I like it so much. Its just WWII SMG fun, at its purest. In the end, I am glad i discovered this type of firearm, even if i could only afored semi-auto kit builds for my collection. They are still enjoyable to take out shooting, and most of their parts do date back to 1940s Britan.
Sorry this thread took so long to complete. As with the one on the Uzi, life kept getting in the way. Its done now though, and I hope you have enjoyed reading it. I definitely enjoyed researching and writing it. If you think this is a wall-o-text, you should see the information i had in my notes that i decided not to include. So much out there on the good o' Sten Machine Carbine.