During World War I, the German designer, Hugo Schmeisser had developed the MP-18 submachine gun, which his employer, the German manufacturer, Theodor Bergmann, started to produce in his factory by 1918 and about 10,000 were made before the war ended soon after. This weapon was the first practical submachine gun used in combat (though not the world's first submachine gun). Even though this weapon was only used for a short period of time, it gained such a reputation that the treaty of Versailles explicitly specified submachine guns among the list of weapons banned from the German military. One of the weaknesses of the MP-18 was its snail-drum magazine, so shortly after World War I, Hugo Schmeisser improved the design to use box magazines. Bergmann continued to make this weapon in secret for a few years after World War I ended and about 25,000 (or 35,000 depending on who you ask) were made in total. However, since Bergmann could no longer manufacture these weapons openly, they finally sold the improved design to the Swiss company SIG (Schweitzerische Industrie-Gesellschaft) in 1920, which started to sell the improved design as the Bergmann Submachine Gun. SIG sold Bergmann models of various calibers to several countries, such as China, Japan and Finland, between 1920 and 1927, when they finally stopped making these weapons.
The Finnish army wasn't interested in this weapon initially, but the Finnish Civil Guard (Suojeluskunta) were interested and bought about 1000 Bergmann submachine guns from SIG in the early 1920s. Around this time, a talented Finnish weapon's designer named Aimo Lahti thought the Bergmann was too expensive and the reliability could be improved as well. He made his first prototype in 1921, a model that fired the .32 ACP cartridge and was about 11.8 inches (30 cm.) long and it was handmade by a blacksmith in Viiala. By 1922, he had a real working prototype (M/22) which was built by a factory with machine tools (Leskinen & Kari in Tampere). He tried to get the Finnish military interested in this weapon, but they refused because it wasn't really production-ready yet, but some members of the Keski-Suomi regiment did express interest in it. Therefore, in 1924, Lahti established Konepistooli Oy (Translation: Machine Pistol Corp.) with members of this regiment, Captain V. Korpela, Lt. Y. Koskinen and Lt. L. Boyer-Spoof. (Korpela had to leave the company later, as he was trying to sell the submachine gun to other countries without permission of the other shareholders).
Lahti continued to make improvements and by 1924, the Finnish Defence Ministry got interested and bought about 100 submachine guns. At this time, Konepistooli Oy needed every sale they could get. By 1926, the M/26 model was introduced, which used a very unique 36-round magazine, that was not used in any weapon before then.
The Suomi M/26 submachine gun. Click on the image to enlarge.
Note the unique curved magazine shape of this weapon. The M/26 cost about half the price of the Bergmann, but it did not sell well. It also had some feeding issues and shared some of the weaknesses of the original MP-18 and Bergmann. The stock was also not strong enough for military use. Lahti succeeded in fixing these issues in his new model, the M/31.
One of the improvements in the M/31 was to remove the excess room in front of the bolt (which sometimes caused cartridges to turn sideways during loading) to prevent jamming problems. A quick release mechanism allowed the barrel to be rapidly replaced. An improved muzzle brake reduced the muzzle climb during shooting in automatic mode. The most important change was the cartridge used, as the M/31 was designed to use the 9x19 mm. parabellum cartridge that many other countries were using at that time. Because of the 9x19 mm. cartridge, two new magazines, a 20 round box magazine and a 40 round drum magazine were developed as well.
The Finnish Defence Ministry were very interested in the M/31, but Konepistooli Oy lacked the facilities for mass production. Luckily, an engineer named Oscar Ostman was a personal friend of Aimo Lahti. Ostman was the CEO of a Finnish company called Tikkakoski Rauta ja Puuteollisuusyhtio (Translation: Tikkakoski Iron and Wood Products Ltd.), which originally was a metal workshop, but also had experience in making firearm parts (like barrels for rifles and machine guns) for the Finnish military. Interestingly, the major shareholder of Tikkakoski was a German weapons dealer named Willi Daugs. Tikkakoski bought the rights to produce the M/31 from Konepistooli and called it the Suomi KP/-31 (KP standing for Konepistooli (i.e. Machine Pistol) and 31, since it was manufactured in 1931).
The Suomi KP/-31 submachine gun. Click on the image to enlarge.
Image licensed under Creative Commons Attribution-Share Alike 3.0 Unported license by Mbeesb
The 20 round box magazine could actually be filled with upto 25 cartridges, but it wasn't too reliable when it was filled with 25 cartridges and it was felt that 20 rounds was too little capacity for the magazine and so, the manufacture of these stopped by 1939. The 40 round drum magazine also had some reliability issues, but its bigger problem was loading it -- cartridges had to be inserted into the drum with the cartridges standing on the tips of the bullets. The slightest vibration could cause the cartridges to fall on their sides inside the magazine and the user would have to dump all the cartridges out and start from the beginning again. Luckily, Lt. Y. Koskinen, who was one of the other founders of Konepistooli Oy, came up with the design of an improved 71 round drum magazine, without Aimo Lahti's finding out until it was ready for production. This 71-round magazine was the most well known and successful magazine design of the KP/-31. In fact, the Soviets were so impressed by it, that they cloned it for their PPSh-41, as we saw in the previous article. A 50 round quadruple column casket box magazine of Swedish design was also made for the KP/-31, but this magazine proved vulnerable to small dents and therefore, it was mostly issued with the 71 round drum magazine.
When the Winter War started, Finland only had about 4000 of these in service, but after the Winter War ended and as the Continuation war started, production had ramped up to about 1400-1500 weapons made per month. Finnish soldiers used this weapon with deadly effect against the Soviets. This weapon showed what a useful weapon the submachine gun was to modern armies and other military forces were quick to adopt this concept as well.
The KP/-31 had a number of interesting features about it. It was a blowback action using Advanced Primer Ignition. This means the firing pin ignites the primer of the cartridge, before the bolt stops into battery. The bolt's momentum keeps the cartridge case locked in the chamber until the peak chamber pressure from the gases has dropped to a safe level. Unlike later submachine gun designs, this was largely built by machining the components and therefore took longer to manufacture. For instance, the receiver was machined from a solid forged steel block. The steel used for the barrel and receiver was the best Swedish made chromium-nickel steel that was available during that time. Some of the original barrels for the pre-1931 models were made by Birmingham Small Arms (BSA) in England, but Tikkakoski started making their own barrels by 1931, and many were also supplied by the precision competition rifle manufacturer, Joonas Matarainen. Interestingly, for a submachine gun, it had a quick-detachable barrel and a barrel jacket, features that are usually only found on heavy machine guns. Also, unusually for a submachine gun, each KP/-31 was supplied with two barrels (primary and spare barrel). Each pair of barrels were machined to a very high degree of precision, so that they both had very similar dispersion and point of impact. This allowed the user to quickly change barrels in the middle of a firefight without needing to adjust the sights afterwards! Even though the weapon had a high rate of fire (around 900 rounds per minute), it was surprisingly well-balanced and controllable.
How accurate was it, the reader asks? Factory testers would shoot 10 shots at a target in semi-automatic mode, using a simple bench rest and all 10 shots would have to hit the bullseye of the target at 100 meters distance. The following two images show the results of tests conducted by the Finnish Army during official acceptance tests.
Click on the image to enlarge.
The diameter of the center circle (the ten circle) of both targets has a diameter of 4 inches (25 mm.). The shots were made from a distance of 100 meters using a simple bench rest. The left image shows 15 rounds fired at this distance using semi-automatic mode. The right image is using a 50 round magazine fired in a single long burst at the same distance, using full-automatic mode. As you can see, the test in semi-automatic mode has a 1 MOA accuracy (and it wasn't uncommon to get sub MOA groups either). In the right image, it can be seen that 48 out of the 50 shots are within the smallest or second smallest circle.
Unlike most other submachine guns, the sights were adjustable to 500 meters distance, which was a bit optimistic, but the weapon was easily capable of hitting targets at 300 meters distance. It was much more accurate than every other submachine gun of that period (and probably even now). In the dense Finnish forests, the range of the KP/-31 was more than adequate and the large drum magazine and fast rate of fire meant it could spray more lead than practically any other automatic weapon of that time.
One more interesting feature was the cocking handle (charging handle). It was a non-reciprocating design (i.e.) it does not move back and forth when the gun is fired (similar to that of a bolt-action rifle). The user only pulled it back once after loading a new magazine into the weapon, to cock it initially, and after that the charging handle would not move at all. Because of this feature, it didn't have a bolt handle slot and therefore, was less likely to let mud and snow enter into the firing mechanism, which led to greater reliability.
A couple of variants were made to be used inside bunkers and tanks. These had thinner barrel shrouds and no shoulder stock, using a pistol grip instead. These were designed to be used in enclosed spaces where it was necessary to fire through narrow slits. On these variants, the sights located on the left side of the weapon, to make it easier to aim through a slit. The tank version was designed with a special barrel shroud that was permanently attached to the firing port of a 6-ton Vickers tank (the T-26E model). If it became necessary for the crew to abandon the tank, the weapon could be easily removed from the firing port and normal barrel shroud attached to it to use as a regular submachine gun.
One more variant was from 1942. This variant had an "improvement" to add a muzzle brake. This model was called the KP/-31 SJR (SJR from the word "suujarru", the Finnish word for "muzzle brake" or "compensator"). This increased the length of the weapon by 55 mm. Aimo Lahti didn't like the new compensator because the oblique front end of the barrel jacket was found to be as efficient as the compensator. On the other hand, the compensator caused trouble in cold frosty weather, as powder and primer residue would get trapped in the chamber and the resulting sticky mass of condensed water, carbon and salts would stick to the breech bolt and cause misfires. On top of that, the compensator reduced the muzzle velocity a bit as well. This "improvement" enraged Aimo Lahti so much that he attempted to find out who was responsible for this change and have him charged in a military court, but to this day, the identity of the genius responsible remains a classified secret!
One more variant was from 1942. This variant had an "improvement" to add a muzzle brake. This model was called the KP/-31 SJR (SJR from the word "suujarru", the Finnish word for "muzzle brake" or "compensator"). This increased the length of the weapon by 55 mm. Aimo Lahti didn't like the new compensator because the oblique front end of the barrel jacket was found to be as efficient as the compensator. On the other hand, the compensator caused trouble in cold frosty weather, as powder and primer residue would get trapped in the chamber and the resulting sticky mass of condensed water, carbon and salts would stick to the breech bolt and cause misfires. On top of that, the compensator reduced the muzzle velocity a bit as well. This "improvement" enraged Aimo Lahti so much that he attempted to find out who was responsible for this change and have him charged in a military court, but to this day, the identity of the genius responsible remains a classified secret!
The Finns used this weapon against Soviet forces with great success and showed the world how submachine guns could be used in warfare by regular troops and not just as defensive weapons by truck drivers and tank crews.
The Suomi KP/-31 had superior firepower, excellent accuracy and very high reliability. On the downsides were its weight, high production cost and slower production rate. Finnish infantry soldiers liked this weapon a lot. It was also manufactured under license in Sweden, Denmark and Switzerland and also used by other countries such as Norway, Israel, Germany etc. In 1938, the British Army conducted tests on many different submachine gun models and their Ordnance Board concluded that the KP/-31 was the best weapon by far. However, since the Finns were battling the Soviets at that time, the British concluded that they might not be available for purchase and opted for the Thompson submachine gun instead.
Incidentally, Norway used them until the 1980s and the Vatican Swiss Guard used them until the 1970s (as the Swiss made version of the KP/-31, the Hispano Suiza MP43/MP44). The tank version of the KP-31 (which was described 4 paragraphs above) was still in service in Finland through the 1980s, despite the fact that the T-26E tank that it was designed for, was decommissioned by Finland in 1959!
