For one reason or another, both projects sank into oblivion during the spring of 1943. Factory No. 9’s design bureau never revisited the problem of installing heavy gun systems on a KV tank chassis. It concentrated on tank guns and systems for medium SP guns.
Petrov’s abandonment of the idea of installing a BR-2 and U-3 in the KV-14 did not mean that the issue was dead. As the saying goes, nature abhors a vacuum, and Factory No. 172 took Factory No. 9’s place with a vengeance. This turn of events was logicaclass="underline" the Perm enterprise had developed and was the only producer of the ML-20. The factory’s design bureau also explored numerous heavy artillery projects using that gun-howitzer’s tipping parts.
In March 1942, Factory No. 172’s design bureau began developing a 203 mm corps-level mortar. S. P. Gurenko spearheaded work on the system, which received the factory designation M-4. Red Army artillery had no system like that, making it an enterprising project. In developing the M-4, Gurenko’s designers were guided by the following:
An artillery gun firing a powerful shell is an absolute necessity for destroying large enemy fortifications.
The 203 mm howitzer model 1931 (B-4) performs that mission.
However, the use of this howitzer in maneuver warfare is hindered by its relatively high weight and low mobility.
In addition, because these howitzers are no longer being produced, the front does not currently have the numbers of heavy howitzers that it needs.
There are three prototype light-weight 203 mm corps-level howitzers—the M-40, the BL-39, and the U-3. They were designed to meet GAU operational requirements, which stipulate that each should weigh over a third less than the B-4 howitzer.
Because they are more maneuverable than the B-4 howitzer, these models would be better suited to modern warfare.
If any of these prototypes is accepted into the inventory, however, industry would face the problem of starting up an entirely new production line with all of the problems that entails, and those problems would be exacerbated by the wartime conditions. We cannot, therefore, expect a significant number of these machines to be produced in the near future.
The factory has an idea—supply a 203 mm howitzer that would be much lighter than the B-4 howitzer but would fire the same shell with a range of 8–10 km and would be based on the existing 152 mm gun-howitzer model 1937 (ML-20).
This howitzer, which would exhibit all the advantages of maneuverability and would be easy to mass-produce, would also have a sufficiently long range with the standard shell.{1}
In other words, Factory No. 172’s design bureau decided to take a shortened B-4 howitzer barrel and mount it on the ML-20 gun-howitzer carriage. Unlike Factory No. 8’s design bureau, which in October 1942 submitted only a conceptual design for a U-3 howitzer barrel on an ML-20 carriage, its competitors went much further. On May 14, 1942, the GAU’s Artillery Committee issued a finding on the M-4 project that basically endorsed the idea. The first tests on the mortar took place from May 18 through 20, 1942, and involved an experimental carriage. A ballistic barrel made from the prototype M-40 203 mm howitzer’s tube and the B-30 152 mm gun’s breech mechanism was used for the test.
A small number of modifications were made to the ML-20 design for mounting the heavier and more powerful barrel. The top carriage was given a special fitting for attaching a purchase block and tackle for use in retracting the barrel for travel. The bottom carriage was given a limiter associated with the purchase block and tackle, also a small modification; and a trail was added to the design. The equilibrator was given a stronger spring, and the makeup of the SPT&A kit was altered somewhat.
A monobloc barrel was manufactured for the mortar in August 1942. The breech mechanism was taken from the B-4 howitzer with minor modifications. The tube was manufactured from scratch. The barrel was equipped with a massive slot-type muzzle brake to reduce recoil. The length of the barrel to the muzzle face was 3693 mm (18.2 calibers); with the muzzle brake added it was 4328 mm. For ammunition, the mortar used the normal shells for the B-4 203 mm howitzer. The shorter barrel reduced the muzzle velocity to 353 m/s.
According to factory records, the M-4 mortar was developed to destroy field fortifications. The carriage enabled it to fire at angles from -2° to 65° and at ranges of 3 to 9.8 km. The M-4 weighed 8300 kg in travel position and 7500 kg when configured for combat, which exceeded the same figures for the 152 mm gun-howitzer by less than half a tonne.
The M-4 began factory testing with a monobloc barrel on August 25, 1942, and 71 rounds had been fired by September 18. It fired a total of about 100 rounds during testing. In addition, it underwent towing tests between September 18 and October 21. In all, the M-4 traveled 400 km. No warping was detected during the tests. One defect did show up, but it was not due to design flaws. While climbing a slope, the prime mover towing the howitzer moved backwards and struck the left trail, bending the ammunition tray.
On November 5, 1940, the M-4 system was shipped to GAU’s Gorokhovets Artillery Scientific and Technical Experimental Test Range (GANIOP) (Ilyino Gorkovskaya Railroad Station) by order of GAU chief Col. Gen. Yakovlev. The technical documentation for the system accompanied it. The tests lasted from January 21 to March 13, 1943. During that time, the mortar fired 456 rounds and traveled 458 km over various types of snow-covered roads. The road tests were not without incident: while avoiding an oncoming vehicle on February 27, the Voroshilovets prime mover that was towing the mortar performed a maneuver that caused the M-4 to fall in a ditch, resulting in some damage.