152 mm howitzer-gun M1937 (ML-20)

The 152 mm howitzer-gun M1937 (ML-20) (152-мм гаубица-пушка обр. 1937 г. (МЛ-20)), was a Soviet gun-howitzer. The gun was developed by the design bureau of the plant no 172, headed by F. F. Petrov, as a deep upgrade of the 152-mm gun M1910/34, in turn based on the 152-mm siege gun M1910, a pre-World War I design by Schneider. It was in production from 1937 to 1946. The ML-20 saw action in World War II, mainly as a corps / army level artillery piece of the Soviet Army. Captured guns were employed by Wehrmacht and the Finnish Army. Post World War II, the ML-20 saw combat in numerous conflicts during the mid to late twentieth century.

Description
The ML-20 was officially classified as howitzer-gun, i.e. an artillery system which combines characteristics of a howitzer and (to lesser extent) of a gun and therefore can be used in both roles. This universality was achieved by wide range of elevation angles and by using separate loading with 13 different propellant loads. The gun was fitted with both telescopic sight for direct fire and panoramic sight for an indirect one. For ballistic calculations and meteorological corrections a special mechanical device was developed. The device, called meteoballistic summator, consisted of a specialized slide rule and a pre-calculated table. After World War II similar devices were introduced for other types of guns.

The barrel was either monobloc or built-up. Some sources indicate that a third type - with loose liner - also existed. To soften recoil, a large slotted muzzle brake was fitted. The breechblock was of interrupted screw type, with forced extraction of cartridge during opening. A safety lock prevented opening of the breechblock before the shot; if there was a need to remove a shell, the lock had to be disabled. To assist loading when the barrel was set to high elevation angle, the breach was equipped with cartridge holding mechanism. The gun was fired by pulling a trigger cord.

The recoil system consisted of a hydraulic buffer and hydropneumatic recuperator. Each held 22 litres of liquid. Pressure in the recuperator reached 45 atm.

The carriage was of split trail type, with shield and balancing mechanism, leaf spring suspension and steel wheels with rubber tires (some early production pieces received spoked wheels with solid tires from M1910/34). During transportation the barrel was usually retracted. The gun could also be towed with the barrel in its normal position, but in this case the transportation speed was limited, about 4–5 km/h (compared to 20 km/h with barrel pulled back). The gun could be set up for combat in 8–10 minutes. The carriage, designated 52-L-504A, was also used in the 122-mm gun model 1931/37 (A-19).

Development history
Among other artillery pieces the Red Army (RKKA) inherited from the Imperial Russian Army a 152-mm siege gun M1910, developed by Schneider. The gun was modernized twice in 1930s, resulting in the 152-mm gun M1910/30 and the 152-mm gun M1910/34. However, its mobility, maximum elevation and speed of traverse still needed improvement. In 1935-36 the No. 172 Plant in Motovilikha tried to continue the modernization works, but the Main Artillery Directorate (GAU) insisted on more significant upgrade.

Consequently, the design bureau of the plant developed two guns ML-15 and ML-20. While the former project was initiated by GAU, the latter started as private development; the team working on it was led by F. F. Petrov. Both guns used barrel and recoil system of the M1910/34. The ML-20 also inherithed from the older gun its wheels, suspension and trails.

The ML-15 reached ground tests in April 1936, was returned for revision and was tested again in March 1937, this time successfully. The ML-20 went through ground tests in December 1936 and through army tests next year. After some defects (mostly in carriage) were eliminated, the ML-20 was recommended for production and on 22 September 1937 it was adopted as 152-mm howitzer-gun model 1937 (152-мм гаубица-пушка образца 1937 года (МЛ-20)).

It is not clear why the ML-20 was preferred. The ML-15 was lighter (about 500 kg less in combat position, 600 kg in traveling position) and more mobile (maximum transportation speed 45 km/h). On the negative side, the ML-15 had more sophisticated carriage (however, the final version of the ML-20 carriage incorporated some features of the ML-15). Some sources claim that the choice was made because of the economic factor - the ML-20 was more similar to the M1910/34, meaning less arrangements for production.

Production history
The gun was in production in 1937-46. 6,884 guns were manufactured and about 4,000 ML-20S barrels for using in self-propelled guns SU-152 and ISU-152. The ML-20 was eventually replaced by the D-20 152 mm gun with identical ballistics, which entered production in 1956.

Smaller production rates toward the end of the war were caused by two reasons. First, most of the barrels produced in these years were ML-20S. Second, after Germans fielded heavy tanks the plant was ordered to increase production of A-19 122 mm guns instead of some ML-20.

Red Army
The ML-20 was originally intended for corps artillery. Together with the 122-mm gun A-19 it formed a so-called "corps duplex". In 1940-41 there were three types of corps artillery regiments:
 * With two battalions of ML-20 and one of either A-19 or 107-mm guns (a total of 24 ML-20s).
 * With two battalions of ML-20 and two of either A-19 or 107-mm guns (a total of 24 ML-20s).
 * With three battalions of ML-20 (a total of 36 ML-20s).

Soon after the outbreak of Operation Barbarossa, the corps artillery was eliminated (as rifle corps themselves were eliminated). It was restored later in the war. The new corps artillery regiments were supposed to be armed with 122-mm guns or 152-mm howitzers, but some memoirs mention that the ML-20 was also used.

From 1943 the gun was employed by artillery regiments of armies. Such regiment had 18 ML-20s. Guard armies from early 1945 had artillery brigade with 36 ML-20s.

The ML-20 also used by artillery regiments (24 pieces) and brigades (36 pieces) of the Reserve of the Main Command.

It was primarily used for indirect fire against enemy personnel, fortifications and key objects in the near rear. Heavy fragments of the OF-540 HE-Fragmentation shell were capable of piercing armour up to 20–30 mm thick, making a barrage dangerous to thinly armored vehicles and to some extent to heavier armoured ones as the fragments could damage chassis, sights or other elements; sometimes a close expolsion caused damage inside a vehicle even though the armour remained intact. Direct hit of a shell often resulted in tearing away a turret of a medium tank or jamming it in case of a heavy tank.

The gun was also equipped with armour-piercing shells for direct fire against armoured targets. Although not an ideal anti-tank gun because of its large size, slow traverse and slow rate of fire, in 1943 the ML-20 was one of a few guns effective against new German tanks. For example, the glacis armour of the heavy Tiger I could be penetrated from about a kilometer.

The first combat use of the ML-20 was in the Battle of Khalkhin Gol, in limited numbers. It also saw combat in the Winter War against the Mannerheim Line fortifications. The gun proceeded to be used throughout the second world war.

Excellent characteristics of the gun, including reliability and ease of maintenance, allowed it to remain in service with the Soviet Army for a long time after the war.

Other operators
In the early stage of the German invasion of the Soviet Union hundreds of ML-20 were captured by Wehrmacht. The gun was adopted by Germans as 15.2 cm KH.433/1(r). From February 1943 Germans manufactured an ammunition for the gun.

The Finnish Army captured 37 guns of the type in 1941-44 and received additional 27 from Germany. These guns were adopted as 152 H 37. A number of barrels were mated with carriages of A-19 to create 152 H 37-31. While the gun was generally liked, the Finnish Army didn't possess enough prime movers suitable for towing such a massive piece. As a result, some of the guns were assigned to coastal artillery. Two pieces were captured back by the Red Army. A number of these guns were modernized in 1988 and as of 2004 still remain in reserve as 152 H 88-37. (all of the 152 H 88 series are being withdrawn as of 2007).

After the war the ML-20 was widely exported to Warsaw Pact allies and to many states in Asia and Africa (in some of those states the gun still remains in service). It was adopted by Egypt and Syria and therefore saw action in Arab-Israeli conflict. In 2002 a TV documentary featured ML-20 employed by the Afghan Northern Alliance forces against the Taliban fighters; it seems likely that the guns were initially supplied to the Najibullah's regime.

Variants
The barrel was manufactured in two variants - monobloc or built-up. Some sources indicate that a third type - with loose liner - also existed.

Except the basic variant, the only variant to reach mass production was the ML-20S, developed for use in self-propelled guns, with differently placed controls for easier operation in small enclosed compartment.


 * ML-20 with bag loading - In 1937 the Main Artillery Department decided - for economic reasons - to modify medium caliber guns to use bag loading instead of cased loading. An experimental ML-20 piece was built in 1939; the trials were unsuccessful.
 * ML-20SM - Modified ML-20S with identical ballistics. Muzzle brake was removed. The gun was mounted in the experimental ISU-152 model 1945, of which only one unit was built.
 * BL-20 - ML-20 with cast breech and wedge breechblock. Was developed by OKB-172 (sharashka of the NKVD) in 1946.
 * ML-20 with M-46-type carriage - ML-20 with a modified carriage, resembling in construction a carriage of the M-46 130 mm gun. In 1950 an experimental piece was built and tested.
 * 152 H 88-37 - A Finnish modernization program
 * 152 mm haubico-armata wz. 37/85 - A Polish modernization, tires from KrAZ 255B truck, new compressed air cylinder and automotive lighting increased the road speed of the towed gun from 20 to 50 km/h and allowed the gun to be towed on public roads.

Self-propelled mounts

 * SU-152 - KV-1s heavy tank chassis, in production from February to December 1943, 670 units built.
 * ISU-152 - IS-1 heavy tank chassis, in production from November 1943 to 1946, 3,242 units built.
 * ISU-152 model 1945 - a single prototype utilizing IS-3 heavy tank chassis.

Summary
The ML-20 was one of the most successful Soviet artillery pieces of World War II. Its characteristics positioned it between classical short-range howitzers and special long-range guns. Compared to the former, the ML-20 has better range (e.g. the German 15 cm sFH 18 had range of 13.3 km), which often allowed it to shell positions of enemy artillery while remaining immune to enemy fire. Its advantage over the latter was in weight and cost, and therefore in mobility and production rate. For example, the German 15 cm K 39 with range of 24.7 km weighed 12.2 tons and only 61 pieces were built; of the excellent 17 cm K 18 (23.4 t, 29.6 km) 338 pieces were manufactured; lighter 10.5 cm sK 18 (5.6 t, 19.1 km) was more common (2,135 pieces) but its 15 kg shell was much less powerful than a 44 kg shell of ML-20. German attempts to produce an analogue to the ML-20 were unsuccessful. The 15 cm sFH 40 was never produced due to construction defects; the 15 сm sFH 42 had insufficient range and only 46 pieces were built. In 1943 and 1944 Wehrmacht announced requirements for a 15 cm howitzer with a range of 18 km, but none reached production.

Of other guns with more or less similar characteristics, there were French 155 mm guns model 1917 and 1918 with longer range, but some 3.5 tons heavier (as was the US 155 mm Long Tom). The Czechoslovakian howitzer K4 (used by Germans as 15 cm sFH 37(t)) was about 2 tons lighter, but with range more than 2 km shorter and only 178 pieces were built. The British BL 5.5 inch (140 mm) Medium Gun probably had the closest characteristics; weight slightly over six tons and range of 18100 yd with an 82 lb shell.

The main shortcomings of the ML-20 were its weight and limited mobility. As the experience of the ML-15 project suggests, the gun could be made somewhat lighter and more suitable for high-speed transportation. The use of a muzzle brake can be seen as a minor flaw: while softening the recoil and thus allowing the use of a lighter carriage, a muzzle brake has the disadvantage of redirecting some of the gases that escape the barrel toward the ground, where they can raise dust, potentially revealing the gun position. But when the ML-20 was developed, muzzle brakes were already a common design element in artillery pieces of that class.

Interesting facts about ML-20

 * On 2 August 1944 the ML-20 howitzer-gun No. 3922 became the first gun to shell German territory in the second half of World War II.
 * Loader of the ML-20 had to carry 40 kg shells alone.
 * The 41st artillery brigade - one of the first brigades to be equipped with the ML-20 - after a number of reorganizations became a training center for soldiers and sergeants of the Russian Army Missile and Artillery Forces. There is a memorial ML-20 piece in one of the center's parks.
 * A small number of operational ML-20 howitzer-guns is still present in the Russian Army ordnance depots.
 * 152 mm HE-Frag projectiles OF-540, initially developed for the ML-20, are still in Russian Army service and can be fired from modern 152 mm ordnance pieces.
 * ML-20 howitzer-guns were towed by heavy artillery tractors Voroshilovetz and Komintern; these vehicles produced by the same Kharkov locomotive plant that developed the T-34 tank and were equipped with the same V-2 diesel engine.

Surviving pieces
ML-20s are on display in a number of military museums. Among other places, the gun can be seen:
 * In the Central Armed Forces Museum and at the Poklonnaya Hill, Moscow, Russia.
 * In the Museum of Artillery and Engineering Forces, Saint Petersburg, Russia.
 * In The Artillery Museum of Finland, Hämeenlinna, Finland.
 * In the IDF History Museum (Batey ha-Osef), Tel Aviv, Israel.
 * In Poznań Citadel, Poland.
 * In the Lubuskie Military Museum, Drzonów, Poland.
 * In the US Army Ordnance Museum, Aberdeen, Maryland, USA.