The Suomi Model 1931, often referred to as KP/-31 or the M/31, was a Finnish submachine gun that saw service in Finland during World War II.
The Suomi was a high quality submachine gun designed by Aimo Lahti, a famous Finn and weapon designer, in the late 1920s. The gun's barrel was longer than most contemporary submachine guns, increasing its range well beyond its competitors. The weapon's rate of fire was exceptional, if excessive. Much like some machine guns, a device allowed the barrel to be rapidly removed from its sleeve and replaced during over-heating, useful for coping with the extremely high rate of fire.
The KP/-31 was a very heavy weapon, around 7 kilograms fully loaded. Most of the weapon was of wood construction, although the trigger assembly, barrel and receiver were machined from high-quality steel.
The safety was located just in front of the trigger guard and doubled as a fire selector. The weapon was capable of semi-automatic and fully automatic fire. The Suomi's rear sights were adjustable for up to 500 meters, and its front sights were adjustable horizontally for wind.
The weapon was not without its faults, however. It was rather expensive to produce, requiring steel and other valuable wartime resources. It was also known for its high recoil. Later in the war, muzzle adapters were added to fix this issue. This adapter, however, was known to fill up with ice and snow during cold weather. Lahti himself disapproved of the adapter, believing that no further recoil compensation was needed.
The Suomi KP/-31 was adopted by the Finnish army shortly after its production began. Various models were produced, some with foregrips and drum magazines, a feature later copied by the Soviet submachine guns. 
The Suomi Model 1931 played a major role with the Finnish Army in fighting the Soviets in the Winter War, as a vicious close-quarters weapon easily applied for the deep, rapid strikes into dense Soviet infantry formations. Captured Model 1931s were said to have inspired the PPD-40 and PPSh-41 designs.
The Suomi was used internationally, and was manufactured under license in Denmark, Sweden and Switzerland. A large number of stock KP/-31s were exported to other countries including Bulgaria, Croatia, Estonia and, most numerously, Germany. In German hands, most of the Finnish submachine guns were issued to the Waffen-SS.
A number of Suomis also made their way into the Spanish Civil War. Around 300 Suomis were confiscated from fleeing Spanish rebels in France, which were later captured again by German forces after the invasion of France in 1940.
In Finland, the Suomi was so successful that it remained in service until the 1980s.
- M/26: The original prototype developed by Aimo Lahti. This version featured a long, curved 36-round magazine and had a different stock design. It was the first of the series to feature the quick-exchange system for the barrel. Only 100 were produced between 1925 and 1926.
- M/31: The base model for the series. This version had a redesigned stock and came with the 71-round drum magazine as standard. Later in the war, many were fitted with muzzle brakes to deal with recoil. About 80,000 M/31s were produced in total.
- M/31 SJR: The "Suujarru" (Finish: Muzzle Compensator) variant. This later model featured a muzzle brake for recoil.
- M/31 "Korsu-Suomi" Bunker Version: This version was specially modified for use in the extreme close-quarters environments of bunkers. The entire stock was removed and replaced with a pistol grip, and a slimmer barrel was installed, which flattened at the tip like a duck's bill. On some models, the sight was fixed to the side of the weapon. The new thinner barrel was 435mm long, which allowed the Suomi to be fired through narrow vision slits.
- M/31 Tank Version: This variant was modified for use as a hull-mounted machine gun in Finnish T-26E light tanks. The weapon could be easily removed by the crew for use as regular submachine gun. It was very similar in design to the bunker version.
- M/33: A prototype model built for Persia. This variant featured a top-mounted box magazine and a bi-pod. Another version had a pistol grip, and a detachable rear stock. These were built in very small numbers and never delivered.
- M/37: A Swedish variant of the Suomi chambered in the country's standard 9x20mm Browning cartridge. It used a 56-round box magazine.
- M/37-39: Another Swedish variant re-chambered to the original 9x19mm Luger. This model can be easily distinguished from Finnish models by its unique stock, which was more akin to the M/26 version. It also had a much shorter barrel. These models used the Finnish 50-round dual staggered magazine. A number of these were exported by Sweden to Norway, Denmark, Indonesia, and Egypt.
- M/37-39F: Swedish designation for the standard KP/-31. A total of 190 were exported to Sweden.
- M/41: A Danish license-built model of the Suomi M/31. Most of these were disarmed or discarded after the German invasion of Denmark and the forced disbandment of the Danish military.
- Mp.43: Swiss designation for pre-muzzle brake Suomi M/31s. Switzerland received about 100 in 1942.
- Mp.43-44: A licensed Swiss-built version of the M/31. The production was taken on by the Hispano-Suiza company, who built approximately 22,500 Mp.43-44s. This version had a simplified rear sight featuring two settings; 100 meters and 200 meters.
- KF-42: An unlicensed Soviet copy of the Suomi chambered in 7.62x25mm Tokarev. The Soviets referred to these as "Карело-Финский 42" (Karelian-Finski 42, Russian: Karelo-Finnish 42). This version was simplified and much cheaper than the Finnish versions. Only very few were produced.
- ↑ The Encyclopedia of Weapons of World War II, pg.251
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 http://world.guns.ru/smg/fi/suomi-m31-e.html
- ↑ http://world.guns.ru/smg/fi/suomi-m31-e.html
- ↑ http://www.guns.connect.fi/gow/suomikp5.html
- ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 http://www.jaegerplatoon.net/MACHINEPISTOLS1.htm
- ↑ 6.0 6.1 6.2 6.3 http://guns.connect.fi/gow/suomi2.html
- ↑ The Illustrated World Encyclopedia of Guns, p.243
- ↑ Sotilaskäsiaseet Suomessa 1918 - 1988, part 2, page 267
- ↑ Stéphane Ferrard, France 1940, L'Armement Terrestre, 1998 p. 44.
- ↑ http://img.allzip.org/g/36/orig/8282252.jpg