b) ML-20 is currently being converted from 1250×300 cast wheels to twin KMP 1250×200 wheels, which will reduce ground pressure.
c) The springs in both systems are being strengthened, which poses no difficulty because the ML-20’s spring leaves are simply being enlarged.
<…>
An even more striking result is obtained if, on the one hand, the 152 mm gun model 1935 is combined on the ML-20 carriage with a monobloc barrel and a cast breech having a combat weight of 9000 kg and a travel weight of 9800 kg, and, on the other hand, the same gun on the B-4 203 mm howitzer carriage is combined with a triple-layer barrel with a combat weight of 18,260 kg and a travel weight of 24,000 kg.
Both the service/combat and industrial/economic benefits of this are particularly high.
If the BR-2 152 mm howitzer is placed on the ML-20, then installation of the gun with restricted traverse in a KV tank becomes entirely possible without modifying the tank.
Under our ZIK-20 project to mount it on a KV tank, the installation would use almost the entire cradle and all mounting parts from the ML-20.
The weight of the self-propelled gun would increase relatively little—1800 kg.
In any event, this solution for developing a self-propelled gun is significantly simpler than the approach taken before the war, which involved developing a new and larger chassis for the BR-2 based on the KV tank.{15}
Petrov’s idea appeared a bit bold, to put it mildly, only when viewed from the sidelines. As the experience gained in building the M75 107 mm antitank gun demonstrated, the ML-20 carriage was fully capable of mounting heavier systems. As it later turned out, the same could be said of the M-30 howitzer’s carriage. Of the six proposals, only one became metal. The M-10 152 mm howitzer on the M-30 152 mm howitzer carriage was placed in service with the Red Army as the D-1 howitzer during the summer of 1943. The debut design of Factory No. 9’s design bureau has been so successful that it is still in service in a number of countries.
Factory No. 9’s design bureau had another project in addition to these six gun systems. Petrov’s idea for mounting the BR-2 in the ZIK-20 was translated into a conceptual design. It was no less valuable for being based on the version of the ZIK-20 revised as suggested by the Technical Council of the People’s Commissariat of Arms. The SP gun had a rear hatch, telescopic sight, a rack for stowage of ammunition in a single layer on the floor, and the commander’s cupola from the KV-1S. In addition, the BR-2 installation project included ZIK-20 specifications based on both the KV-1 and the KV-1S.
1. The Technical Council of the People’s Commissariat of Arms and the GAU’s Artillery Committee have approved the ZIK-20 project for mounting the ML-20 152 mm gun-howitzer model 1937 on a KV tank with a fixed (welded) turret. The drawings for this installation are almost finished.
If the BR-2 152 mm gun is mounted on the ML-20 carriage, the barrel of the BR-2 gun with a muzzle brake made of armor steel can easily be placed on the ML-20’s cradle.
The barrel would only require modification of the cast breech. Its weight would increase and its weight distribution would change. The modification would be required to achieve balance and provide the needed firing angles.Only the profile of the recoil throttling rod would change as compared with the tank-mounted ML-20.
2. This installation would result in a self-propelled gun with the following characteristics:
| No. | Parameter | GAU’s operational requirement | ML-20 152 mm gun-howitzer model 1937 | BR-2 152 mm gun model 1935 |
|---|---|---|---|---|
| 1. | Caliber, mm | 152 | 152 | 152 |
| 2. | Shell weight, kg | 49 | 43.5 | 49 |
| 3. | Muzzle velocity, m/s | 880 | 600 | 880 |
| 4. | Charge type | Bagged | Separate loading | Bagged |
| 5. | Weight of entire SP gun using: | |||
| a) KV tank, t | Not more than 65 | 53 | 55 | |
| b) KV-1S tank, t | Not more than 65 | 46 | 48 | |
| 6. | Number of shells | 47 | 30–40 | 40–47 |
| 7. | Displacement of center of gravity relative to a KV tank, mm | – | 360 | 390 |
| 8. | Armor thickness, mm: | |||
| a) Glacis | 60 | KV: 75, KV-1S: 60 | KV: 75, KV-1S: 60 | |
| b) Side | 60 | KV: 75, KV-1S: 60 | KV: 75, KV-1S: 60 | |
| c) Turret | 60 | 75 | 75 | |
| 9. | Traverse angle | +/-4° | +/-6° | +/-5° |
| 10. | Elevation angle | +15°, -3° | +15°, -3° | +15°, -3° |
| 11. | Distance barrel extends beyond vehicle, mm | – | 1700 | 3000 |
| 12. | Line-of-fire height, mm | – | 2190 | 2206 |
3. As the table and the drawing of the KV tank installation show, the BR-2 installation differs little from the ML-20 installation in the same tank, but it is more than twice as powerful.
It is necessary to accept the greater extension of the gun barrel forward of the tank as compared with the ML-20 because we are dealing with such a powerful barrel, which, together with the muzzle brake, is about 8000 mm long.
If there is a need to develop a self-propelled gun of this type, this solution is the most practical, the simplest, and the most constructive and technically correct of all the alternatives.
4. Since the BR-2 152 mm gun shares a carriage, cradle, breech, and recoil mechanisms with the 203 mm howitzer, placing the BR-2 on the ML-20 tank cradle also offers the opportunity, as needed, of placing the B-4 203 mm howitzer’s monobloc barrel on this cradle with similar modifications and, of course, it would be even easier to place the U-3 203 mm howitzer in a tank.{16}
The idea of mounting the BR-2 on an ML-20 cradle was entirely feasible; moreover, similar projects were implemented in 1944. On the other hand, it was much more important towards the end of 1942 to complete work on the ZIK-20 superstructure, and the situation with that project was far from rosy. The transfer of SP gun projects to Factory No. 9 delayed work on the design even more. The situation was aggravated by the fact that the model shop at the new factory was less capable. As a result, the full-scale wooden model of the ZIK-20 was not finished until December 15.