We hereby submit data comparing the KV-14 SP gun with the OBM-50 project:
| No. | Parameter | Unit | KV-14 | OBM-50 | |
|---|---|---|---|---|---|
| 122 mm gun | 152 mm gun | ||||
| 1. | Caliber | mm | 152.4 | 121.98 | 152.4 |
| 2. | Shell weight | kg | 43.5 | 25.2 | 43.5 |
| 3. | Muzzle velocity | m/s | 650 | 1000 | 880 |
| 4. | Range at 23° | km | 12 | 21 | 18 |
| 5. | Armor thickness penetrated at 1500 m | ||||
| 0° | mm | – | 195 | 195 | |
| 30° | mm | – | 160 | 160 | |
| 6. | Traverse angle | degrees | +/-7 | +/-7 | +/-7 |
| 7. | Basic load on vehicle | Rounds | 16 | 16 | 16 |
| 38 (modified fighting compartment) | |||||
| 8. | Rate of fire | Rounds/min | 1.5 | 2 | 1.33 |
| 9. | Vehicle weight with gun and basic load | t | 47 | 48 | 48 |
Having submitted the conceptual design, OKB-172 is continuing work on the project, which enables OKB-172 to produce the engineering drawings immediately upon receipt of your findings.{12}
Moreover, three versions of the OBM-50 were submitted, all with the same muzzle velocity—1000 m/s. The first version of the gun (referred to as “Version A” in documentation), which was discussed in a memorandum, had a barrel 8566 mm (70.2 calibers) long and a maximum pressure of 2670 kg/cm². According to its design, this version resembled the A-19 120 mm gun very closely. Two additional versions of the OBM-50 were submitted on October 28. They were referred to as “Versions I and II.” Version I’s barrel was shortened to 8390 mm (68.7 calibers) and its chamber lengthened to 990 mm. Version II featured many more differences. The length of its barrel was reduced to 7430 mm (60.9 calibers); it also had a 990 mm chamber. Its maximum pressure was increased to 3000 kg/cm². The increased pressure required a new design for the breech end, which was given a sliding wedge breechblock.
The GAU’s Artillery Committee issued its finding on the OBM-50 conceptual design on November 12, 1943. Interestingly, the Artillery Committee’s 1st Department proposed its own ballistic solution for the gun in addition to these three versions. It had a 7440 mm barrel with a maximum pressure reaching 3200 kg/cm². Thus, there were four projects to choose from:
These ballistic solution versions must be reviewed based on the chief design and tactical requirements for the gun. Three such requirements apply:
1. The barrel must be as short as possible; a barrel length of 55–58 calibers would be desirable.
2. The barrel must have a diameter as small as possible.
3. Manufactured of steel with a strength grade of 0-70.
Rough calculations done by the Artillery Committee’s 2nd Department showed that the pressure in the bore must not ever exceed 3000 kg/cm². For a maximum pressure of 3200 kg/cm², the outer diameter of the barrel would be 400 mm, which would require a breech width of at least 600 mm. A gun that wide would take up one third of the fighting compartment’s width. Next—the thick walls of the barrel parts for a monobloc (about 125 mm) would make heat treatment very difficult, but their hardening characteristics could be achieved if OKhNZM steel is used.
A built-up barrel cannot be introduced under wartime conditions due to the difficulty of manufacture. Moreover, the process for building up barrels is not currently in use at Factory No. 172.
From the standpoint of minimizing the barrel length, maximum pressures under 2800 kg/cm² are also unacceptable because, as research has shown, the barrel length would be more than 67 calibers.
Considering the gun’s special role and its proposed relatively limited fielding (not a high-volume gun), the generally accepted requirements for the costeffectiveness of the use of charges should be discarded. The coefficient of charge utilization for the guns should be established between 80 and 90 by increasing the relative and absolute weight of the charge. This would make the ratio of the powder burn point to the projectile path about 0.75, which would be a reasonable guarantee of complete combustion of powder in the bore. Consequently, the muzzle velocity would not vary due to incomplete powder combustion.
This approach is already being used in modern artillery practice to achieve high velocities and powerful guns. Examples include the 88 mm self-propelled gun model 43/1 (Germany), 75 mm antitank and tank guns (Germany, massproduced guns), and a heavy 3-inch tank gun (United States, prototype).
To ensure that the chamber design is as practical as possible and easy to load, and to ensure that the barrel has the best design shape, the chambrage should be increased to match the diameter of a 152 mm gun, and the calculated charge density should be between 0.73 and 0.75.{13}