The M22 system involves mounting the traversing mechanism of the B-4 203 mm howitzer on the KV-14 tank chassis.
This project entails little design change to the B-4 howitzer or the KV-14 tank chassis. The changes are as follows:
The recoil distance is taken to be 860 mm (the relative position of the rod and counter-rod recorded on short recoil) due to the possible lowering of the line-of-fire height and the need to stabilize the entire system during firing and while traveling.
The recoil resistance needed to be reduced from 88 tonnes to 45 in order to ensure stability during firing.
The design calls for adding a muzzle brake with an absorption coefficient of 48.9% to achieve that.
The design calls for removing metal from the muzzle and giving up the barrel’s normal safety margin in order to keep the center of gravity of the tipping parts on the trunnion axis.
The rest of the B-4 howitzer traversing mechanism remains unchanged.
The howitzer is mounted on a special raised area on the tank hull without inclusion in the mantlet, which made it possible to reduce the weight of the vehicle with its full complement of 26 rounds to 43 tonnes.
To protect the howitzer against bullets and shrapnel, it is equipped with a gun shield attached to the top carriage.
The projectiles are located on special racks above the engine compartment.
Loading is done manually by two or four men using a loading tray.
When loading, the tray is brought to the rack, and one man rolls a projectile into it.
This loading method is simple and enables a good rate of fire, considering the overall dimensions of the system.{6}
The delayed response by Factory No. 172’s design bureau to the requirement to develop a semi-open SP gun mounting the B-4 howitzer can only partially be attributed to installation on the chassis of the SU-152. Almost nothing remained from the original chassis; the initial KV-1S tank was actually wellsuited to modification for this purpose. To achieve the significant reduction in the height of the SP gun, only the tipping parts were taken from the B-4, and the howitzer itself was somewhat modified, of course.
Unfortunately, the situation at that point did not favor the M22. The design was submitted later than its competitor from the TsAKB. Even a week’s delay mattered at this stage. In addition, Factory No. 172’s design team had made changes to both the design of the howitzer and that of the SU-152 chassis. The addition of a massive muzzle brake to an SP gun that was mainly intended to fire from cover clearly worked against the M22. In addition, the muzzle brake would raise a great deal of dust during direct fire, which would give away the gun’s position and make it difficult for the crew to fire the weapon. The outcome was inevitable: The technical Council of the People’s Commissariat of Arms ordered further work on the M22 to cease.
In late November 1943, Factory No. 172’s design bureau tried for a second time to push through a heavy SP gun project. On the 27th, the design bureau sent materials on the M24 203 mm howitzer project led by A. Ya. Drozdov to the GAU’s Artillery Committee. It involved mounting a barrel with the ballistics of the B-4 203 mm howitzer on the ML-20 152 mm gun-howitzer’s carriage. It also proposed that consideration be given to mounting a barrel with the ballistics of the BR-5 280 mm mortar (designated the M25) on the same carriage. A spot was also found at the end of the memorandum for mention of a self-propelled version of the two systems:
In addition, the traversing mechanisms of both the M24 and the above-listed options for mounting a barrel would also be beneficial for mounting on a vehicle for the same reason.
To illustrate that, the attached materials include a sketch showing the traversing mechanism of the M24 system mounted on a KV-1S or KV-14 tank chassis.{7}
The version of the M22 with an M24 gun was assigned the factory designation M26, and the version mounting the M25 was designated the M27. According to the memo, they planned on using the KV-1S or IS as the chassis. However, the only difference between the M26 and the M22 was that the M26 had a new gun system and no shield. In addition, the project envisaged placing the BR-2 152 mm gun on the same carriage. This was the M23 152 mm gun, which was under development at the same time.
On 15 December, the GAU’s Artillery Committee reviewed the M24 howitzer project. It was decided to proceed with that project and turn it into metal while at the same time rejecting the version with the barrel from the BR-5 280 mm mortar. The M26 and the M27 were not even mentioned. The TsAKB’s brainchild thus was finally left with no competitors.
There was some backstage maneuvering going on with the transfer to TsAKB of KV-1S and SU-152 vehicles in repair status that Grabin had requested to serve as chassis for new systems. It became clear in 1943 that production of the KV-1S and SU-152 was coming to an end. The TsAKB chief therefore requested one IS tank and the drawings for it in addition to an order for two KV-1S and one SU-152. GABTU sent a report about that to Beria on October 11:
In accordance with letter No. NV-10/5070, dated October 6, 1943, from Comrade Ustinov, the People’s Commissar of Arms of the USSR, I hereby report the following:
1. Two type KV-1S tanks without armament and turret may be transferred from repair status to the Central Artillery Design Bureau of the People’s Commissariat of Arms, since new KV-1S tanks are no longer in production.
2. One SU-152 self-propelled gun in repair status may also be transferred.
3. General-view drawings of the hulls of both the KV-1S tank and the SU-152 may also be transferred by the People’s Commissariat of the Tank Industry, for which transfer People’s Commissar Comrade Malyshev has given his consent.
The People’s Commissariat of the Tank Industry may only transfer an IS and the drawings for it after testing of the tank is complete and the drawings are brought up to date, because only prototypes of IS tanks are currently being manufactured. They will not complete testing and go into production until November of this year.{8}
Two KV-1S tanks and one SU-152 were sent to TsAKB in November. Because the bureau had begun working on new systems, in a letter dated November 10, Grabin tried to get a KV-85 in place of one KV-1S. The request was granted: TsAKB tested an S-34 100 mm gun on the KV-85. The design bureau did not make use of the SU-152 because subsequent projects involved the ISU-152.
The situation with development of the semi-open SP guns equipped with the B-4 howitzer was also quite interesting. The fact is that the design bureau was working on systems without having clear specifications for them. With only a vague assignment in hand, TsAKB initially considered several options for the SP gun chassis. The situation was unchanged in October; TsAKB decided to undertake the project without approved specifications. The GAU’s Artillery Committee finally drew up an operational requirement “for 152 mm and 203 mm auxiliary-propelled guns for the Artillery Reserve of the High Command (ARGK)” on November 16. It was signed by GAU chief Col. Gen. Yakovlev on November 18: