7. Minimize the line-of-fire height. A height of 2170 mm has been achieved in this project, exceeding the line-of-fire height of the 76.2 mm gun in the KV tank with small turret by only 70 mm.
8. Facilitate assembly in a cooperative work environment to fulfill the task.
We mounted the ZIK-20 tank gun on a KV tank using a welded fixed turret of prismatic shape.
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The turret wall thickness was taken to be 75 mm. Thus, in our understanding, we took the KV-7 tank as the base chassis; that is, not some prototype of the tank, but in the belief that any KV tank with a fixed turret is a KV-7 type tank.
The total weight of the “KV-7” tank together with the ZIK-20 system will be approximately 53 tonnes, i.e., it will be about 4 tonnes heavier than the KV tank with a small turret.
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IV. Main features of the ML-20 howitzer system
1. The ZIK-20 152.4 mm tank howitzer was produced using the tipping parts of the ML-20 152.4 mm howitzer with a modified cradle, replacing the elevation and traversing mechanisms, and establishing constant recoil by attaching a counter-rod and removing the variable recoil mechanism and muzzle brake.
2. The design changes listed in paragraph 1 were made entirely by modifying the field gun into a tank gun, adding armor, and bringing the system into balance. The installation involves removal of the muzzle brake for reasons explained below.
3. The ballistics of the ZIK-20 152.4 mm tank gun are the same as for the 152.4 mm howitzer model 1937.
4. The ZIK-20 152.4 mm tank gun fires time-fuzed, high-explosive, and concrete-piercing shells.
5. The relatively powerful munitions, the strong armor (up to 90 mm in thickness), and the sight mechanisms that support both direct and indirect fire enable the ZIK-20 gun on the KV tank to be used for the following purposes:
a) Destruction of bunkers and trenches both at short range and from cover;
b) Engagement of enemy mechanized infantry units when operating as self-propelled artillery;
c) Suppression of enemy artillery fire.
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VIII. General observations about the project
In the absence of a specific Operational Requirement for mounting the ML-20 on the KV tank, this project is based on existing performance specifications for similar models, with due consideration given to features specific to the ML-20 system.
Concerning the modifications made to the artillery system and several items inside the tank, the following can be said in conclusion:
1. We deem it necessary to remove the muzzle brake from the system; retaining it for a tank gun would be a mistake.
2. All changes to the cradle were made for the sole purpose of converting the field gun to a tank gun.
3. A practical loading test may show that the loading tray is unnecessary. Given the system’s small angles of elevation (15°) and the low height of the gun’s axis above the floor, and without a cradle extending beyond the breech face, which usually hinders loading, the crew will find loading easier without a tray, which would take time to readjust.
4. From a strength standpoint, the recoil mechanism parts are surely reliable (considering that firing will be done without a muzzle brake).
In practice, we adopted a constant recoil equal to the short recoil of the ML-20, thus eliminating the variable recoil mechanism. The front cradle cover could also be removed or the counter-rod attachment altered. However, resolution of this issue depends on the number of vehicles manufactured. If the number produced is small, the old hatch cover will be used. If the number produced is large, the counter-rod should be modified, which will reduce and simplify a number of cradle and recoil mechanism parts, and then the armor for the cradle can be made more compact.
An issue with the sight needs to be addressed.
We do not believe the decision to select and install the standard model 1927 sight is straightforward.
To give such a powerful system the capability of both direct and indirect laying, we believe it should have both a conventional sight and a panoramic sight such as the TOP tank gun sight.
The use of a TOP-type sight would make it possible to meet the important requirement for reducing the size of the sighting slit.
To resolve this problem, the factory needs to acquire drawings for current models of elongated tank sights.{11}
Thus, rather than simply installing an ML-20 in the hull of the KV-7, Factory No. 8’s design bureau was essentially proposing a new SP gun based on the KV-1. In addition, the ZIK-20 howitzer being mounted in the SP gun required even more modifications than the ML-20 under Project U-18. The SP gun’s superstructure was 17 centimeters higher than the KV-7, and the vehicle created by Factory No. 8’s design bureau weighed more than a KV-2. Given the completely new superstructure, the KV-7 hulls in Chelyabinsk were unusable.
The ZIK-20 project was reviewed at a meeting of the Technical Committee of the People’s Commissariat of Arms on September 15. Projects of the armor department at the Bauman Institute of Mechanical Engineering were also reviewed at the meeting. They included self-propelled mounts for the I-13-52 57 mm antitank gun and the B-4 203 mm howitzer. As designed, the SP gun with the B-4 had two ZIS-5 engines, and it had a top speed of all of 6.5 km/h.
The ZIK-20 was the meeting’s main topic of discussion. Factory No. 8 design bureau chief F. F. Petrov reported on the project. His report included an explanatory note, drawings, and engineering analyses. The idea of eliminating the muzzle brake, which would give away the SP gun’s position during firing, was approved, but a suggestion was made to use a loading tray like the one on the SG-122 SP gun. Although Petrov raised the telescopic sight issue, the design did not include one (in contrast to the U-18, which had included a TOP from the outset.) In addition, mounting the ML-20 on the ZIK-20 rather than on the U-18 would locate the gun further forward, which threatened to increase the load on the front road wheels.
The layout of the fighting compartment generated many more questions from those present at the meeting. The sides of its superstructure were less sloped than those of the KV-7, which reduced the likelihood shells would ricochet. Although the height of the fighting compartment from the floor to the roof was 1895 mm, the crew would have to work in very cramped conditions. The double-row rack for shells on the floor took up 395 mm and would be very inconvenient to use. In addition, the attachment of racks to the sides of the superstructure increased the risk that the basic load would explode if the superstructure were struck by enemy shells. The decision to locate fuel tanks along the sides of the vehicle was questioned. A number of questions regarding the effectiveness of the fighting compartment ventilation and defense of the vehicle from the rear, which did not even have a submachine gun port (in contrast to the U-18, for which a stern-mounted DT machine gun was planned).
The exchange of views on Factory No. 8’s design led to the following conclusion:
1. The ZIK-20 project for a self-propelled gun mounting the ML-20 152 mm gun-howitzer model 1937 on a KV tank with a fixed turret proposed by Kalinin Factory No. 8 was particularly interesting as a practical solution to the problem of developing a heavy self-propelled gun with good armor protection to serve as a bunker buster.