The M-17 was the next-to-last project that involved mounting the 203 mm howitzer in an enclosed superstructure on either the SU-152 chassis or the later ISU-152. The Central Artillery Design Bureau (TsAKB, in Kalingrad, now Korolev) was the last organization to work on a similar project. In October 1943, it developed an enclosed SP gun that would mount either a 152 mm gun with the ballistics of the BR-2 or a 203 mm howitzer with the ballistics of the B-4. The work took place under the S-51 project, but it went no further than the conceptual stage.
After that, the design effort proceeded along two lines. According to the requirements of the Technical Council of the People’s Commissariat of Arms, work began on designs for an open auxiliary-powered system mounting the B-4 203 mm heavy howitzer (this system will be discussed in the next chapter). Attempts to mount a more powerful system on the SU-152 without changing its superstructure continued.
It should be noted that work to install a system other than the ML-20 had begun in the spring of 1943. As mentioned above, the experimental projects involving the SU-152 included mention of installation of an A-19 122 mm corps-level gun-howitzer in an SP gun. The GAU’s Artillery Committee authorized the work, and the Chelyabinsk Kirov Factory and Factory No. 172 were to carry it out. By April 29, 1943, the Chelyabinsk Kirov Factory had received one A-19 system, and work was underway to install it and develop the ammunition rack. A prototype SU-152 mounting the A-19 gun was expected by May 10, but work stalled for various reasons.
Meanwhile, in the summer of 1943 the Red Army came face-to-face with an opponent more formidable than the Tiger. Among the new armored vehicles employed by the Germans on the Kursk Salient, the 8.8 cm PaK 43/2 Sfl L/71 Panzerjäger Tiger (P) SP gun, better known as the “Ferdinand,” stood out. The glacis and superstructure of the heavy German tank destroyer boasted armor 200 mm thick, which meant it could survive fire even from corps-level artillery.
To say that the Soviet military took note of the Ferdinand greatly understates their reaction. The appearance on the front of this vehicle, which had had a production run of 90 units, sparked a number of projects. Suffice it to say that the “Object 701” heavy tank (the future IS-4), for which specifications were developed in November 1943, was created specifically to counter the Ferdinand. Work on the D-25 122 mm tank gun was also related to Ferdinand Porsche’s creation, although, in all fairness, development of that gun had begun earlier.
The M75 107 mm antitank gun reappeared among a list of tank, antitank, and self-propelled artillery projects dated September 15, 1943. One reason for the revival of that project, which had been abandoned in 1942, was that the gun was based on the ML-20 gun-howitzer carriage. It was theoretically possible that the idea of putting the M75 on a self-propelled vehicle, which had been buried in the summer of 1941, might have a chance at revival two years later, especially since targets worthy of it had appeared. However, the return to the 107 mm antitank gun project was very short-lived. In the same letter that mentioned the M75, GAU Artillery Committee chief Maj. Gen. Khokhlov suggested dropping a number of projects and replacing them with more promising ones:
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Work should stop on other projects in this group for the following reasons:
1. The ZIS-3 76 mm antitank gun with a muzzle velocity of 850 m/s differs little from the ZIS-2 57 mm antitank gun in terms of its armor-piercing effect.
2. Upgrading the F-34 to a muzzle velocity of 850 m/s would require a new tank turret. A new tank turret would make it possible to mount an 85 mm tank gun that would be more powerful than an upgraded F-34 76 mm gun.
3. The 85 mm, with its muzzle velocity of 1000–1050 m/s, would have almost the same armor-piercing effect as the D-25 122 mm gun. It would be advantageous to put the latter gun into production as it would enable a more rapid resolution of the problem, and because the prospects for increasing the power of the 122 mm are better. In addition, an 85 mm shell at that velocity and against thick armor would be less effective and durable than a 122 mm at the same velocity.
4. Because the M75 107 mm gun, with a muzzle velocity of 1020 m/s, is far from being finished, and because no ammunition is being manufactured for it, it would be better to work on the more powerful 122 mm gun with a muzzle velocity of 1000 m/s.
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The Artillery Committee stresses that, of all of the projects it has approved, the following should be given priority:
1. The upgrade of the F-34 76 mm tank gun chambered for the 76 mm antiaircraft gun model 1931.
2. The upgrade of the D-5 tank gun to give it a muzzle velocity of 900 m/s.
3. The upgrade of the KS-1 85 mm antiaircraft gun.
4. Refinement of the D-25 122 mm tank gun.
Production of these guns can begin quickly to bring our tank guns to the level of similar guns in the enemy’s inventory.
The remaining projects are of lower priority, and work on them must not come at the expense of the higher priority projects listed above.
The Artillery Committee also believes a project not mentioned by the Technical Council of the People’s Commissariat of Arms is a top priority—the completion of work to mount the 122 mm gun model 1931 on the M-30 122 mm howitzer carriage underway at Factory No. 9 under the designation D-2.
For its part, the Artillery Committee proposes the following as high-priority projects:
1. Manufacture of a 122 mm self-propelled gun with a muzzle velocity of 1000 m/s for a projectile weighing 25 kg with armor penetration of 200 mm at a range of 1000 m.
2. Manufacture of the D-25 122 mm tank gun with a sliding wedge breechblock as a more suitable gun for the armored forces.
3. Refinement of the 25 mm antitank gun designed by Comrade Sidorenko of the Artillery Academy and the ChK 37 mm antitank gun as more powerful antitank guns.
4. Manufacture of the 37 mm automatic antiaircraft gun on the SU-76 vehicle as a mobile air defense weapon for tank and mechanized troops.{9}
Thus, instead of reviving the idea of using a 107 mm antitank gun, it was decided to develop a new gun based on the A-19 122 mm gun. That was approved at a meeting of the Technical Council of the People’s Commissariat of Arms and the GAU’s Artillery Committee held that same day—September 15, 1943.
On September 24, the chairman of the Technical Council of the People’s Commissariat of Arms sent a letter to TsAKB chief V. G. Grabin:
By the decision of the People’s Commissar of Arms and the Chief of the Main Artillery Directorate of the Red Army, you are hereby required to begin development of a 122 mm tank gun and self-propelled gun with a muzzle velocity of 1000 m/s for a projectile weighing 25 kg.
You should submit the conceptual design for the gun by October 10, paying particular attention to the development of a ballistic solution and barrel for the gun.
At the same time you submit the conceptual design, send the People’s Commissariat of Arms and the Artillery Committee your draft of the operational requirement for the gun.
The concept is to mount this 122 mm gun on the chassis of a SU-152 122-millimeter self-propelled gun and on the T-34 tank chassis; the latter case will be a self-propelled gun with relatively light armor and will be open at the top. The concept for the 122 mm tank gun is to mount it on an IS tank with specially enhanced armor. The conceptual design should clarify outstanding questions concerning the possibility of mounting this gun on these chassis.{10}