Saturday, 2 September 2017

Swedo-Polish Tank Killer

Germany invented specialized anti-tank artillery before the end of WWI. It is not surprising that German anti-tank guns became some of the best known in their class. A German gun served as the starting point for the famous Soviet 45 mm anti-tank gun. The use of the German 3.7 cm Pak in Spain triggered the development of tanks with shell-proof armour in the USSR. However, another gun gave an even more powerful impulse to the strengthening of tank armour, built in neutral Sweden. Swedish 37 mm anti-tank guns were also used in Spain, and continued serving until the end of WWII.


Anti-tank bestseller

Located in Karlskoga, the Bofors AB company has a long history. It joined the arms industry in the end of the 19th century. The well known Alfred Nobel gave a strong push to the company in this direction. At the time, Krupp was one of Bofors' chief competitors, as they were supplying artillery to the Swedish army.

The relationship between the two companies took a different shape after WWI. According to the Treaty of Versailles, Germany could not produce or sell new armaments, and cooperation with Bofors was an excellent workaround. The first joint venture between Krupp and Bofors dates to 1919, and, year after year, the cooperation between yesterday's competitors increased. By the end of the 1920s, Krupp owned a third of Bofors' stock.

A captured Armata przeciwpancerna 37 mm wz. 36 Bofors on trials in the USSR, winter 1941.

Both sides benefited from this cooperation. Krupp took advantage of a loophole, which let it skirt around the Treaty of Versailles and develop new armament. The Swedes drew an even larger benefit. The German arms giant was forced to share its developments, and Bofors now had access to foreign markets. Unlike another Swedish arms manufacturer, Landsverk, which only managed to begin tiny shipments of the L-60 tank towards the end of the 1930s, Bofors' name echoed throughout the world, as did the sound of its weapons.

Swedish guns were supplied to many countries around the world, and were no worse than their foreign analogues. On the other hand, some of Bofors' guns were so good, that they themselves became subjects of imitation. Even in the field of tank development, Bofors was luckier than the tank-specialists at Landsverk. Tank turrets from Karlskoga were widely used on Polish 7TP tanks and Finnish Vickers Mk.E tanks.

The same gun from the left. The arrow points to a beam that locks the trails in travel position.

One of the results of this cooperation was a 37 mm anti-tank gun. Both companies worked on anti-tank guns since 1921, but then, the 37-47 mm guns proved unsuccessful. Work in the early 1930s turned out much better results.

The history of the Swedish anti-tank gun is directly connected with the Swedish tank program, which was also influenced by the Germans. Landsverk 10 and Landsverk 30 tanks received turrets that were obviously descended from the turret used on the German Leichttraktor Krupp and Leichttraktor Rheinmetall. The 3.7 Tak gun, designed by Rheinmetall, also migrated to Swedish tanks. However, the Swedes introduced large changes at this stage. The gun, indexed 37 mm K. fm/32, had a vertical sliding breech, unlike the German horizontal design, and had differences in other small parts. Nevertheless, the roots of this gun were definitely German.

The gun from the rear. The pads on which the gunner and one of the loaders lay during battle, are visible.

It was clear that the purpose of a dedicated tank gun was rather narrow. Bofors designers produced a towed infantry variant, the first prototype of which appeared at the same time as the tank gun, in 1932. The towed gun also had the index 37 mm K. fm/32. Largely, it was an adaptation of the tank gun for infantry use, specifically in an anti-tank role.

Towing trials behind a truck.

The design of the gun was vastly different from the German 37 mm gun. The first version had one trail with a special pad. The gunner lay down on it in battle. China bought 13 of these guns in 1933.

An improved version of the gun appeared in that same year, featuring a sloped gun shield. There were now two trails. Denmark became interested in the new gun. In 1933, the Madsen company purchased a license to produce the 37 mm K. fm/32.

A year later, Bofors began trials of a modernized version of the gun, indexed 37 mm K. fm/32-34. Several versions of the gun shield were trialed. As a result, the Swedish army adopted a version with a flip-down lower section, known as the 37 mm infanterikanon m/34.

A gun with the shield removed, showing the aiming mechanisms and mount.

Poland showed interest in the Swedish anti-tank gun in 1935. It was light and powerful, and easily usable in a tank, which was not surprising, given its history. Poland purchased a license to produce the gun under the name Armata przeciwpancerna 37 mm wz. 36 Bofors. 300 Swedish-produced Bofors guns were also purchased.

The SMPzA company (Stowarzyszenie Mechanikуw Polskich z Ameryki) in Pruszków, south-west of Warsaw, was tasked with the production. Polish guns differed little from Swedish ones. The biggest difference was altered tires.

Aside from equipping its own army with licensed guns, the Poles exported them themselves. For example, 24 guns were sent to Spain, where Republicans used them during the Civil War.

Dissembled body of the gun.

Bofors received other orders after the Polish one. 12 guns were bought by Holland in 1937. Interestingly enough, the British made a large order. 250 guns were purchased for Sudan, where they were indexed Ordnance QF 37 mm Mk.I.

The Swedish army bought only twelve 37 mm infanteriekanon m/34, which entered service in 1935. The order for the modernized 7 mm pvkan (pansarvärnskanon) m/38 was much larger. You can familiarize yourself with a gun of this type, preserved in the Swedish Arsenalen museum, here. In total, 491 guns of this modification were purchased, starting in 1939.

Close up view of the muzzle brake. Its presence was a liability, since it revealed the position of the gun during firing.

Bofors' product was the most numerous exported anti-tank gun by the start of WWII. Poland alone had 1200 of these weapons. This kind of saturation had an effect on the flow of battle. The Armata przeciwpancerna 37 mm wz. 36 Bofors and its tank version became the biggest threat to German tanks in September of 1939. As a result of the Polish campaign, the Germans lost 819 tanks (236 of them irreparably), after only a month of fighting. Swedo-Polish anti-tank guns proved to the Germans that protection from 20 mm autocannons, developed after fighting in Spain, was insufficient. It was clear that the armour of tanks needs to get thicker.

Better late than never

The Red Army first met the Swedish guns during the Polish campaign, which began on September 17th, 1939. No conclusions about the combat effectiveness of the Armata przeciwpancerna 37 mm wz. 36 Bofors were made. The reason was simple: there were almost no battles where the gun was used.

A small number of anti-tank guns was captured in battle. There is information that the guns were used by the Red Army in 1941, but the author has no specific details. It is known that Polish guns were used in the Red Army, including in armoured trains, but these were 75 mm guns (effectively, former French 75 mm mod. 1897 field guns).

Semiautomatic mechanism from the right.

The Soviet forces encountered Bofors guns several months later. These guns became the most dangerous enemies of the tanks that were storming the Mannerheim Line. From November 30th, 1939, to March 1940, 955 Soviet tanks were knocked out by artillery fire, 368 tanks were lost irreparably. A large number of these losses came specifically from Swedish anti-tank guns. Dents from their shells were found in the armour of KV and T-100 tanks used in battle.

As a result of the fighting in Finland, applique armour was installed on T-26 and T-28 tanks. At the same time, work on a light tank with shell-proof armour began, which led to the creation of the T-50. The Soviet military clearly saw that the time of tanks with bulletproof armour was at an end.

Upper and lower carriage from the front. One can clearly see that the horizontal traverse flywheel is located on the lower carriage.

Even though the Swedish gun was Finland's main anti-tank weapon, the nearly identical Polish gun was not studied. The Main Artillery Directorate of the Red Army (GAU KA) only began to study it in the winter of 1941.

In February-March of 1941, the Gorohovets Artillery Scientific Research Proving Grounds (ANIOP) performed trials of the captured "37 mm Polish anti-tank gun mod. 1936". Aside from a detailed technical description, the ANIOP specialists performed full gunnery trials consisting of 384 shots. In addition, mobility trials were performed, with the gun being towed by a ZIS-5 truck over 200 km.

Trails with the pads removed.

As a result of the trials, the following conclusions were made:

"1. The 37 mm Polish anti-tank gun mod. 1936 is among the newest weapons of modern foreign armies.
...
3. Our 37 mm anti-tank gun mod. 1930 is less satisfactory than the Polish 37 mm anti-tank gun mod. 1936 compared to modern requirements for anti-tank guns.
...
5. The design of the Polish 37 mm anti-tank gun mod. 1936, as well as several individual components (shield, level trigger, recoil brake stem, recoil break grease configuration), as well as the gunpowder propellant, are of interest to our designers and industry."

The wheels were the main difference between the Polish and Swedish anti-tank guns. The Swedish 37 mm pvkan m/38 tires left a characteristic HOHOHO pattern in the dirt.

Meanwhile, the 37 mm mod. 1930 gun, which was inferior to the Swedish design, had nearly vanished from the army. Even the mass produced 45 mm mod. 1932 anti-tank gun was no longer the most modern domestic system. At the end of 1936, a decision was made to produce the modernized 53-K system, known as the 45 mm anti-tank gun mod. 1937, which had the same penetration as its predecessor, but an improved design.

Optical sight.

The penetration, a much more important parameter for an anti-tank gun, was lacking in the Swedo-Polish gun. In comparative trials, the Soviet 45 mm mod. 1932 gun could penetrate 40 mm of armour at 800 meters at normal, and the same thickness at 400 meters at 30 degrees. This was already not enough for the GAU, and, as a result, work was underway on the 57 mm ZIS-2. Shells fired from the 45 mm anti-tank gun mod. 1932 showed satisfactory robustness.

As for the Soviet and Polish anti-tank guns, their results were much more humble. In the same conditions, they could only penetrate 16-20 mm plates, and the performance of the Soviet gun was better. After penetration, shells fired from the Soviet 37 mm mod. 1930 gun remained intact, and shells fired from the Polish one broke up.

Shells from the а) Polish б) Japanese and в) Soviet anti-tank guns. The likeness between Soviet and Polish shells is not surprising, since they share a German common ancestor.

In their conclusions, ANIOP specialists judged both 37 mm guns as unsatisfactory. Their shells, according to Soviet testers, were only useful against light tanks with about 20 mm of armour. Presumably, the Germans made the same conclusions. After the Polish campaign, PzII tanks gained an additional 20 mm of armour in the front, increasing the total thickness to 35 mm.

Anti-tank guns used by the Swedish army had more impressive characteristics. Trials in Finland showed that they could penetrate 40 mm of armour at 30 degrees from 300 meters. This was achieved by a modernization of the gun, which increased the muzzle velocity. The Swedish shells were also higher quality than the ones produced in Poland.

Gun shield after penetration trials.

Despite the praise of the design of the Swedo-Polish gun, the weapon did not draw much attention from Soviet designers. The Red Army already had a good 45 mm gun, and the 57 mm ZIS-2 that was under development had a radically different design.

The only people to continue development of the 37 mm pvkan m/38 were the Swedes themselves. The result was the 57 mm 3 pvkan m/43 cannon, which looked like an enlarged 37 mm pvkan m/38. This gun had much more impressive penetration characteristics, and was used by the Swedish army for a long time. Unlike its predecessor, it never saw combat.

Captured German guns, Courland, spring of 1945. This shot shows very rare guns: Dutch 3.7 cm fodfolkskanon M.1937,  also known as 3.7 cm PaK 157(d).

37 mm Swedish and Polish guns lasted for a long time on the field of battle. Having evaluated the quality of the guns, Finland bought 10 Polish guns from the Germans, but stopped at that. Presumably, the Finns obtained the same penetration results during trials as Soviet testers. As a result, they preferred to set up their own production, purchasing a license from Bofors. In total, Finland produced 355 guns, which were used until the beginning of 1944.

Romania was more eager to buy former Polish guns. In total, it received 556 cannons, which were actively used in battle on the Eastern Front. The guns that the British bought for Sudan also saw combat. They were often installed on trucks used by special units (LRDG) on raids on North Africa. Finally, the Germans themselves used the captured guns under the designation 3.7 cm Pak (p). The guns were last used in 1945, alongside analogous Dutch guns, which were called 3.7 cm PaK 157(d). However, by that time, these guns were hopelessly obsolete.

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