Staff Writer (Updated: 11/1/2012):
The submachine gun was a battlefield component that was still evolving by the time of World War 2. To the West, there arose such names as the American M1 Thompson, the British Sten Gun and the German MP43 series and each proved, in their own way, the value of such compact firearms capable of dealing "machine gun-type" shock unto the enemy. Perhaps one of the lesser-known contributions to the SMG field for most readers became the excellent Suomi Konepistooli KP/-31 (also known as the "Suomi m/31" or "Kpist m/37") series of Finnish design - its origins dating as far back as 1922. The KP/-31 was developed from the preceding "KP/-26" which was introduced in 1926 and itself based on a prototype known simply as the "M-22". The KP/-26 was chambered for the 7.63mm Mauser cartridge and sported a noticeably curved magazine. However, the series only saw very limited production and the arrival of the KP/-31 sent the KP/-26 out to pasture.
The KP/-31 was introduced in 1931 and design of the weapon was attributed to Aimo Lahti and Lieutenant Y. Koskinen. Serial production was handled out of the state-owned Tikkakoski facility to which some 80,000 examples were ultimately delivered. The KP/-31 would go on to see extensive service throughout the Finnish actions against the Soviet Union and played well up to its inherently strong and robust design. Production would cease in 1953 but the KP/-31 itself would exist in frontline service into 1998 - a true testament to its excellent arrangement and construction.
Externally, the KP/-31 appeared as any other conventional submachine gun system of the period. It sported a single-piece wooden body with an integrated ergonomic stock making up the rear hand grip. The metal portion of the receiver - containing the necessary internal working components - was dropped within the wooden body. The trigger unit was underslung from the wooden receiver near the hand grip and consisted of a curved trigger assembly and wire-thin trigger guard. Magazines (or in some cases "drums") were inserted into the feed mechanism along the bottom of the forend. The barrel protruded a distance away from the gun body and was shrouded in a slotted heat shield. Iron sights were provided in the form of a rear notch and a front post. The KP/-31 was chambered for the 9x19mm Parabellum cartridge and could fire from a 20-, 36-, 40- or 50-round detachable box magazine (or "casket" boxes) as well as an impressive 71-round "drum". The firing action was a conventional straight blowback system to which 750- to 900-rounds-per-minute could be loosed. Overall weight was just over 10lbs with a running length of 34 inches, the barrel measuring just 12 inches. Maximum range was out to 500 meters.
The KP/-31 was produced to a higher quality than most other weapons of World War 2 which, in the long run, proved one of the few limitations of the series - it was simply too expensive and time-consuming to produce in the large numbers required of war time. The design arrangement also made the Suomi KP/-31 rather heavy for a submachine gun but this in no way detracted from its battlefield usefulness for the weapon proved to be very accurate in the role - especially at close quarters. Construction included machining and forging of the vital metal parts and this made for a highly robust and reliable weapons while the rest of the form was completed in readily available wood. The firing mechanism included a spring which was fitted within the bolt and allowed the design to be shortened in length to a more compact form. The bolt handle (with a fixed firing pin) was set to the rear of the gun body and was essentially managed by the internal mechanics in much the same way that a manually-operated bolt-action would be. The action proved wholly sound in practice and added to the excellent qualities of the KP/-31.
Some 4,000 KP/-31 were in circulation by the time of the Soviet invasion of Finland. The offensive only prompted a stout resistance on the part of the Finns and the war machine ramped up to fulfill increased demands. The KP/-31 was issued to Finnish infantry squads in limited numbers, however, largely due to their lack of availability. Nevertheless, the m/31 soldiered on in good form and dealt the invading Red Army a nasty surprise. The Finns proved experts in their knowledge of the home country and utilized the environment to their advantage - in a few instances even laying waste to entire Soviet Army elements. Captured Soviet weapons were reused whenever possible and the Finns ultimately dealt blows to both Soviet invincibility and national pride, embarrassing the Red Army on the world stage. Despite their larger numbers, the Red Army was further handicapped by Stalin's "Great Purge" which removed many capable leaders from their posts prior to the invasion - effectively watering down his immense fighting force. The Finns did go on to lose territory in the war but did force an interim peace and maintained their sovereignty. The Soviets lost nearly 127,000 men (including 188,600 wounded), 5,500 tanks and over 400 aircraft in the foray while the Finnish dead numbered 26,000 soldiers. The war would pick up once again in the "Continuation War" which saw the Finns now aided by allied Nazi Germany - the Germans had finally turned their attention against the Soviet Union at this time. The result of this endeavor, however, was a Soviet victory forced on by a stand-still in the fighting. The Finns gained territory in the fighting but subsequently lost these back the Soviets. The Moscow Armistice ended fighting on September 19th, 1944 to which Germany collapsed in May of the following year. The war between the two nations formally ended in 1947 with the signing of the Paris Peace Treaty.
The KP/-31 was developed only into a few known official variants. "The KP/-31 SJR" featured a new muzzle brake which lengthened the overall design some. This variant actually went on to make up some 50% of all KP/-31 produced stocks for the Finns. Another variant was developed and intended for short-ranged, close-quarters use, being completed with a thinner barrel cover and lack of a stock. The stock was replaced by a true pistol grip and this helped to increase portability and compactness of the weapon. A similar vehicle version was also devised for tanker crews.
KP/-31s were also known to have taken part in the 1948 Arab-Israeli War fighting alongside Israeli units. Other global operators went on to include Bulgaria, Croatia, Estonia, Norway, Poland and Slovakia. The German Army of World War 2 ordered some 3,000 examples for their own inventories and these saw heavy use everywhere the Germans fought. Even the Soviet Army made use of captured specimens which clearly showed the type's impressive stature to the enemy. In fact, they went on to copy the KP/-31's 71-round drum magazine for their own wartime PPSh-41 Submachine Gun - the Soviet weapon showcasing an appearance not unlike the Finnish KP/-31. The PPSh-41 would not enter service until 1941.