The 152 mm howitzer-gun M1937 (ML-20) (), is a Soviet heavy 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 breech was equipped with cartridge holding mechanism. The gun was fired by pulling a trigger-cord.
The recoil system consisted of a hydraulic buffer and hydro-pneumatic recuperator. Each held 22 litres of liquid. Pressure in the recuperator reached 45 Bar.
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).
This gun was also mounted in two Soviet assault guns/tank destroyers employed during WWII. These are the SU-152 and ISU-152.
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 inherited the wheels, suspension and trails from the older gun.
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 ().
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 a more complicated 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, thus requiring fewer adjustments for production.