152-mm howitzer-gun ML-20

The 152mm howitzer-gun model of 1937 (ML-20, GAU index – 52-G-544A) is a Soviet howitzer-gun of the Second World War period. This weapon was serially produced from 1937 to 1946 and is still in service with the armies of many countries of the world. It was used in virtually all major wars and armed conflicts of the mid and late 20th century. The most powerful Soviet self-propelled artillery installations of the Second World War – SU-152 and ISU-152 – were armed with this weapon. According to some artillery experts, the ML-20 is one of the best designs of artillery throughout its existence. Even the most restrained assessments acknowledge the outstanding role of the ML-20 in the combat use and development of Soviet artillery in the mid-20th century.

One of the heavy guns of the Red Army's corps-army chain was the 152mm siege gun of 1910 sample, which was obtained from the Russian Army. This weapon was designed by the French company Schneider for the Russian Empire and was used in the First World War and the Civil War. By the 1930s, it was clearly outdated, and two modernizations were carried out in 1930 and 1934. The modernization significantly improved the characteristics of the 1910 gun, but it did not fully meet the requirements of its time, primarily in terms of mobility, maximum elevation angle, and aiming speed. In 1935-1936, the design bureau of plant No. 172 (Perm plant) in Motovilicha considered continuing the modernization of the weapon, but the Main Artillery Directorate (GAU) did not support this work, and it was discontinued. The design of a new weapon began.

Two howitzer-guns were designed at the design bureau of plant No. 172 – ML-15 and ML-20. The group of engineers, led by the famous Soviet designer and armorer F.F. Petrov, worked on the ML-20 project, and initially, work on this weapon was carried out on an initiative basis, while the ML-15 was created according to the task set by the GAU. Both guns had numerous borrowings from the 152-mm gun of the model. 1910/34 - a barrel with a bolt, anti-wind devices. The ML-20 also shared with this gun a common wheel travel, springing and frame, while the ML-15 had these design elements developed from scratch.

In April 1936, the first ML-15 prototype was sent for field testing, but it did not pass and was sent back for improvement. In March 1937, the ML-15 underwent repeat field testing, which was completed without any issues. The ML-20 underwent field testing on December 25, 1936, and the following year, military testing commenced. Following these tests, the ML-20 was recommended for adoption after resolving its mainly carriage-related flaws. On September 22, 1937, the ML-20 was officially adopted and named “152-mm Howitzer-Gun Model 1937.”

From today’s standpoint, the choice of the ML-20 over the ML-15 seems puzzling. A.B. Shirokorad, an author in the field of domestic artillery, believes that the ML-15 had a clear advantage over the ML-20 – it was significantly lighter (600 kg in transport position and 500 kg in combat position) and more mobile (travel speed up to 45 km/h). On the other hand, the ML-15 had a more modern and complex carriage design. Other sources claim that the choice was made for economic reasons – the technological equipment for the ML-20 was already available.

The ML-20 is a howitzer-gun, an artillery system where howitzer properties exceed those of guns. The gun had a relatively modern design for its time, with a carriage featuring sliding legs and a sprung wheel system. The barrel was produced in two different varieties – mounted and monoblock (some sources mention a third option - with an independent tube). The ML-20 was equipped with a piston breech, a hydraulic recuperator of the spindle type, a hydro-pneumatic rammer, and had a separate charge system. The breech had a mechanism for forcibly extracting a fired shell when opened after firing and a safety that locked the breech after loading until a shot was fired. If, for any reason, the gun needed to be unloaded, the safety switch had to be turned on first to allow the breech to be opened. To facilitate loading at high elevation angles, the ML-20 breech is equipped with a cartridge case retention mechanism. The weapon is triggered by pulling the trigger cord. The weapon has a mutual locking mechanism that prevents the bolt from opening if the barrel is not properly connected to the anti-wind devices. To mitigate the recoil, a powerful massive slotted muzzle brake was installed on the ML-20's windage devices and carriage. The rolling stock and the recoil brake each hold 22 liters of liquid, and the pressure in the rolling stock is 45 atmospheres.

One of the distinctive features of the ML-20 is its unique combination of different elevations and initial velocities of the shell, which are determined by the selection of one of thirteen metal charges. As a result, the gun could be used both as a howitzer firing at a high angle with a relatively low shell velocity, and as a cannon firing at a flat trajectory with a high shell velocity. The gun was equipped with both a telescopic sight for direct fire and an artillery panorama for firing from concealed positions.

The carriage with spread legs is equipped with a balancing mechanism and a shield. The wheels are made of metal with rubber tires (some early guns had spoked wheels with rubber tires from the 1910/34 model cannon), and the springs are leaf. Normally, the gun was transported on the carriage with the barrel in the retracted position. The transition time from the traveling position to the combat position was 8-10 minutes. For short distances, the system could be transported with the barrel unretracted at a speed of 4-5 km/h. The ML-20 gun carriage was designed to be normalized, designated 52-L-504A, and was used in the modernization of the 122-mm A-19 gun. The ML-20 was transported by the Voroshilovets and Komintern heavy tracked artillery tractors, which were manufactured by the Kharkiv Locomotive Works, the developer and first manufacturer of the famous T-34 tank. “The Voroshilovets” was powered by the same B-2 engine as the Thirty-Four. The S-65 Stalinets, ATS-712, ATS-59, ATS-59G, URAL-4320, and other trucks were also used to transport the ML-20.