Finnish Precision: Aimo Lahti's Famous M/31 Submachine Gun

By Michael Heidler

The “Suomi” is one of the best-known Finnish weapons of the Second World War. It stands for perfect quality and the highest precision. It also taught the Russian invaders the meaning of fear.

Its creator was Aimo Johannes Lahti. Born in the town of Viiala in 1896, he left school after only six years and got a job in the local glass factory. With his first earnings, he bought an old Russian Berdan rifle. The mechanism of the rifle awakened his interest and, together with a local gunsmith, he began to examine it and to tinker with other weapons in the workshop. After his military service in 1918/1919, he began to work as a gunsmith in the Keski-Suomi regiment in June 1921. At this time, the army was still using German Bergmann MP 18 submachine guns and Finnish copies of the M/20 made under license. Lahti did not like these guns—in his opinion, they were too heavy, unwieldy and too expensive to produce. Therefore, he began thinking about creating his own submachine gun, and in the following months he worked hard on a first draft.

The first prototype he ordered was a downscaled weapon in 7.65x17mm caliber (aka .32 ACP), only about 11.8 inches long and made by a blacksmith in Viiala. The result was no work of art, but it proved that the basic construction that Lahti had designed could work well. In 1922, he ordered the first full-size prototype “M/22,” which was made by the machine workshop Leskinen & Kari in Tampere in 7.65x21mm caliber. The gun had the same rate of fire as the later M/26.

In June 1924, Lahti established the company Konepistooliosakeyhtiö (or Submachine Gun Ltd.) with some officers of the same regiment. The other shareholders of this new business enterprise were Captain V. Korpela, Lieutenant Y. Koskinen and Lieutenant L. Boyer-Spoof. The new company had ambitious plans but very limited finances.

Lahti was working hard on his submachine gun design and making several improvements. Konepistooliosakeyhtiö ordered 100 submachine guns from Ab Tool Oy, and, in August 1924, the Finnish Defence Ministry finally got interested. A few months later, in February 1925, the first batch of 13 submachine guns was inspected by the Ordnance Department of the Finnish Armed Forces. The weapons worked well, but most of the magazines only worked with the individual weapons for which they had been intended. They were not interchangeable. Also, the barrels were already slightly corroded. However, they were accepted even with these minor problems. The Army additionally ordered 10 submachine guns in October 1925 and another 39 in March 1926.

From this production series of about 100 weapons, the majority (over 60) ended up in the Finnish Army, but Suojeluskunta (the Finnish Civil Guard) and the Frontier Guard also bought small numbers. A few weapons of this series were delivered abroad as samples, including five pieces to Estonia. After some modifications in design, the later submachine guns of this production series were renamed as M/26. The Suomi M/26 ended up costing only about 2,200 Finnish marks per piece, while the Bergmann submachine gun was more than twice that price at the time (4,500 Finnish marks per piece).

The Suomi M/26 never got into real use in combat during World War II. The guns were issued to troops doing guard duty at the home front. In 1959, the remaining 57 Suomi M/26s were sold to Interarmco and shipped abroad in 1960. This was one of the darkest moments in Finnish weapons history as, along with the “ordinary” M/26s, all of the one-of-a-kind prototypes were also sold without understanding their historical value.

Aimo Lahti was not happy with the M/26 submachine gun. Its feeding process was not as reliable as it should have been. First of all, he eliminated the excessive hollow space front of the bolt, which had allowed the cartridges to turn sideways and cause jams. After this modification the feeding problems disappeared. The new way in which the barrel was now attached to the receiver allowed the barrel to be quickly replaced if needed. The barrel received a tilted end, which decreased the tendency of the muzzle to climb during full-automatic fire. The jacket around the barrel was modified to a heavier and more robust structure. However, the most important improvement was changing the caliber to 9x19mm Parabellum, one of the most common calibers in the world. The new ammunition required new types of magazine. For this reason, Lahti designed all these improvements plus a new 20-round stick magazine and a 40-round drum magazine in the years 1930 / 1931.

Finally, the interest of the Finnish Defence Ministry was piqued, but there was no factory in which to manufacture the new weapon. Engineer Oscar Östman, who was leading Tikkakoski Rauta ja Puuteollisuusyhtiö (or Tikkakoski Iron & Wood Industry Ltd.) at that time, knew Lahti personally and was very interested in the new Suomi submachine gun. The company had already manufactured gun parts for the Finnish military in the past, so it had enough experience in weapons manufacturing. Tikkakoski bought the rights for mass production as the only manufacturer. This step ended the story of Konepistooliosakeyhtiö—the enterprise had served its purpose.

The name “Suomi” for Lahti’s submachine guns was mentioned for the first time in a Finnish document from 1925. The 9mm model was officially publicized for the first time in the Hakkapeliitta Magazine of the Finnish Civil Guard in September 1930. In October 1931 the Finnish Army decided to place its first order of 100 new Suomi submachine guns in 9mm caliber. Thus, the weapon got the official designation “9,00 konepistooli M/31” (meaning 9mm submachine gun M/31). Suomi submachine guns were available from Tikkakoski in several variants that included a front handle, bipod and two different kinds of selector switches. From these, the Finnish Armed Forces ordered the simplest variant and stuck with it. Other variants ended up being sold abroad only in small numbers.

Before World War II, the orders for the Suomi M/31 were coming in slowly. By 1934, the now more shortly named “Oy Tikkakoski” AB had only delivered 375 Suomi M/31 submachine guns to the Finnish Army. When the Winter War broke out, the demand for these submachine guns from the Finnish Armed Forces did not exceed 4,000 weapons. Most of them were delivered with one spare barrel that was adjusted for the particular weapon. Replacing the barrel of a Suomi M/31 is remarkably easy and obviously designed for soldiers in the field to do when necessary—both the barrel and barrel jacket are locked to the weapon with a simple single switch. Disassembling and reassembling the weapon is also very easy to do.

During the war, Finnish soldiers gave grateful feedback, but the feedback also showed that the Suomi submachine gun was not perfect. The main problem was the tendency of the muzzle to climb during full-automatic fire. The Ordnance Department of the Finnish Army designed a muzzle brake for the M/31. This device was put on all Suomi submachine guns ordered by the Finnish Armed Forces after February 1943. The weapons ordered by the Home Front Troops and those made for export were not equipped with muzzle brakes, even after that date. The muzzle brake was 95 millimeters long with a diameter of 28 millimeters. It added some 55 millimeters to the length of the M/31. The letters SJR (from the Finnish for muzzle brake: “suujarru”) were soon added to the designation of the weapons.

The “Korsu-Suomi” (Bunker Version)

In the summer of 1939 it became clear that the Finnish Army needed submachine guns that could be used for shooting through vision slits and steel observation cupolas of bunkers. Ordinary Suomi M/31 submachine guns were unsuitable because of the bulky barrel jacket and stock. The wooden stock was too long to be operated efficiently inside the confined observation cupolas. So a bunker version with a longer and narrower barrel jacket, pistol grip and sights located on the left side of the weapon was hastily designed. Prototypes were made and tested and the weapon received official approval in September 1939. The model got the name “Korsu-Suomi” (Korsu is Finnish for “fortification”). But then the Winter War broke out and ruined the production plans, as the resources needed for production were needed elsewhere.

Only a few existing prototypes found use with the Finnish troops. Some barrel jackets of bunker versions had already been manufactured and during the war the Finnish troops put them onto their ordinary M/31 models. The resulting weapon was so long that it could hardly be used in observation cupolas or bunkers, but nevertheless it allowed aimed fire through narrow vision slits. During the Continuation War the design of the bunker submachine guns was improved, and in January 1941 500 bunker versions were ordered. The early model made for tests had a 385-millimeter barrel jacket and a specially designed receiver. The mass-production model was 435 millimeters long and used the receiver and trigger group of the ordinary M/31.

The M/31S (Tank Version)

This was a special version made for the Vickers 6-ton tank, later known as T-26E, after equipping the turret with a captured 45mm Russian tank gun and DT machineguns after the Winter War. Those tanks had a firing port specially designed for this submachine gun in their front hull. A special barrel jacket was permanently attached to the firing port. A submachine gun like the bunker version with a pistol grip was put into this barrel jacket from inside the tank. In case of emergency, the crew could easily remove the weapon from the fixed barrel jacket and put on the barrel jacket of the M/31. Now the weapon could be used outside the tank like a standard submachine gun. Only a few dozen tank version M/31S submachine guns were made before the Winter War, and those were the only ones ever made. Russian DT tank-machineguns were captured in large numbers during the war and proved to be more effective than submachine guns when used in armored vehicles. Therefore, the tank version of the Suomi M/31 never went into mass production. The total number was probably only about 40 weapons, of which 31 were left by the summer of 1940. It was intended to install the M/31S in 32 Vickers tanks, but in the end it was only done on 24 tanks. During the Winter War, the Finns captured more than 450 DT tank-machineguns. That was enough to equip all their tanks.

Experiences of the Finnish Army

The Suomi M/31 submachine gun proved to be an excellent submachine gun: It had superb firepower, excellent accuracy, good muzzle velocity and outstanding reliability. The only minus for its users was its weight. The Suomi was remarkably effective when used as an individual weapon issued to the most capable soldiers. It provided excellent short-range firepower in relatively compact form—light machineguns were still large and heavy by comparison.

The Suomi submachine gun was an unusually accurate weapon and had a longer range than most other submachine guns of its time. This was thanks to its quite good ergonomics, rather long barrel (when compared to other submachine guns of that time), soft recoil, powerfully loaded ammunition and good sights. The rear sight is fully adjustable and has—rather optimistically—settings up to 500 meters, while the front sight is drift adjustable. In Finnish forests its range proved to be sufficient. Also, thanks to their 70-round drums and high rate of fire, the Suomi M/31 was able to spray more lead into the air than most light machineguns of that time. The Suomi M/31 became the favored weapon among Finnish troops and was soon distributed to the top soldiers who knew how to get the best out of them.

In autumn of 1942 the Finnish Armed Forces finally had all the submachine guns they had planned on acquiring. But the M/31 had proved so effective that a committee led by General Heinricks went ahead and decided to recommend adding a second submachine gun to all rifle squads as soon as possible. By the end of 1943, enough submachine guns had been manufactured to fulfill the new requirement. Nevertheless, an even more extensive use of the M/31 was suggested, in the form of adding a third submachine gun to every rifle squad, but this could not be implemented before the Continuation War ended and armament production was halted.

Export Deals by Tikkakoski

Before World War II, the orders placed by the Finnish military were so small that capacity for export production existed and Tikkakoski offered the Suomi submachine gun to foreign customers. Starting from 1931, several introduction tours were done by Aimo Lahti and Tikkakoski representatives to other European countries (mainly to Germany and Baltic countries), while projects for sales to more distant countries also existed. Unfortunately, no good export deals could be closed. The only somewhat-large deal following from these efforts was closed with the Estonian Armed Forces, which bought 485 Suomi M/31 submachine guns. The weapons were delivered in September 1938. Smaller deals were also done with customers like the Polish State Police (20 guns delivered in 1933), but the success had to wait until World War II.

During the Interim Peace and the Continuation War, Tikkakoski continued to export Suomi submachine guns abroad. Many of the larger deals were done to ensure getting vital machinery or raw materials in exchange.

Romania wanted to buy 5,000 Suomi M/31 submachine guns in 1942, but the deal couldn’t be closed as the orders of the Finnish military had higher priority. For the same reason, many of the planned export deals listed above could not be executed. In addition, in July of 1942 Finland donated 120 new Suomi M/31 pieces to the German AOK Norwegen (Field Army Command Norway) located in Finnish Lapland. Large numbers of the submachine guns sold to Germany have ended up in the SS and Waffen-SS, as these units were typically equipped with more mixed weaponry than the Wehrmacht (German Army). Both 50-round, four-column “Coffin” box magazines and 70-round drum magazines seem to have been exported with the weapons.

In June 1944 the number of Suomi M/31 submachine guns in use by the Finnish Armed Forces reached more than 52,600 pieces. Losses in battle, damaged weapons and weapons hidden in caches dropped the number of Suomi submachine guns to just a little bit over 40,100 by October 5, 1944. Repairs and weapons returned from weapons caches slowly raised the number. In August of 1951, the Finnish Armed Forces had about 50,100 Suomi M/31 submachine guns and about 300 bunker versions in their inventory. In the summer of 1957, some 53,600 remained under the ownership of the Finnish Armed Forces. In the mid-1950s, new Swedish-designed, 36-round box magazines (which the Swedes had originally introduced for their Carl Gustav M/45 submachine gun) were first bought from Sweden and soon put in production under license in Finland. In the 1950s and 1960s, these new 36-round magazines replaced the old 20-round, 40-round and 50-round magazines, which were mostly scrapped. The new 36-round magazine proved excellent—easy to fill with cartridges, handy and reliable. From the 1960s to 1990s, the old 70-round drums and new 36-round box magazines were the magazine types used with Suomi submachine guns. Assault rifles replaced the Suomi M/31 in most Finnish military units in the 1960s and 1970s, but large numbers still remained in depots as reserves of the Finnish Defence Forces until the late 1990s.

Even today, the Suomi submachine gun has a legendary reputation among Finns. In a poll carried out a few years ago, the Finns voted the Suomi M/31 as the second most important Finnish invention of the 20th century.

Special thanks to Leszek Erenfeicht (Poland) and the archive of the Finnish Armed Forces “SA-kuva.”

This article first appeared in Small Arms Review V22N1 (January 2018)
and was posted online on November 17, 2017


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