Tuesday 6 September 2022

A Round Too Long

Attempts to create fixed ammunition for the 122 mm D-25 gun in 1944-45

It is no secret that the firepower of tanks in WW2 increased dramatically. Armies of the world began the war wither with 37-47 mm long barrelled guns or 75-94 mm short ones. Some tanks even had armament composed only of heavy machine guns or small autocannons. Firepower of heavy tanks increased the most noticeably, particularly in 1943. German heavy tanks settled on the 88 mm caliber, the Americans focused on 90 mm guns with ballistics of the M3 AA gun. The USSR also initially chose an AA gun, an 85 mm one. However, there was a huge leap forward in the summer of 1943. The design bureau of factory #9 under the direction of F.F. Petrov developed the 122 mm D-25 gun by combining the D-2 122 mm gun (its ballistics were very close to those of the A-19) and the cradle of the 85 mm D-5T-85 gun. After trials in the Object 240 experimental tank, it was accepted into service as the IS-2 (IS-122) on October 31st 1943.

The D-25T gun had no alternatives. There were many attempts to replace it, but Petrov's creation survived 3 generations of tanks. It was only replaced with the M-62 on the T-10M.

This was the most powerful gun of of all weapons installed in mass production tanks of the war. There was one issue: the rate of fire. The aimed rate of fire did not exceed 2.5 RPM. Various solutions were tried, including a radical one: the replacement of the 122 mm D-25 with the 100 mm D-10. The latter gun was less powerful and therefore did not stick on heavy tanks. One often sees suggestions to implement fixed ammunition. Do not make the mistake of thinking that the army back then was not as smart as we are now. There were several attempts at developing fixed ammunition for the D-25 gun.

An attempt for the medium class

A new chapter in Soviet tank building history began on November 22nd 1943. People's Commissar of Tank Production V.A. Malyshev signed NKTP order #705ss "On production of an experimental T-44 tank designed by factory #183". This order authorized the development of a completely new medium tank that was accepted into service a year later (on November 23rd 1944). The T-44 went through at least three stages of evolution. The production tank could be considered a fourth stage, since factory #75 in Kharkov had to rework both the tank and the documentation that came with it. Mostly the changes had to do with the chassis of the T-44, but the turret was also altered. Alternative armament was also developed.

100 mm guns were considered the most promising by the fall of 1943, but they were still in development as of February 1944. Malyshev placed his bet on the D-25T gun.

The T-44's armament initially consisted of the 85 mm D-5T-85 gun developed at factory #9 under the direction of F.F. Petrov. This gun was initially developed for installation into the KV-85 and IS-85 heavy tanks as well as the SU-85 tank destroyer. Meanwhile, the first stage of the Battle of Kursk made it clear that this weapon is needed in medium tanks too. The D-5T-85 was installed in the turret of the T-43 tank in September of 1943. Even though the turret was initially developed for the 76 mm F-34 gun, the D-5T-85 fit well. Unfortunately, the T-43 hit a dead end, as the armour had to be increased in addition to the armament, raising the weight of the tank to more than 35 tons. This is why KB-520 developed the T-44, which was lighter than not just the T-43 but also the T-34 thanks to its perpendicular installation of the engine and other novelties. The D-5T-85 gun was a priority for the T-44, but the turret was made with room to grow. There were two variants of the turret: one with a 1600 mm wide turret ring like on the T-43 and one with an 1800 mm wide turret ring. This was excessive for this kind of gun.

Fixed ammunition for the D-25T-44. Surprisingly, this development was not reused a year later.

There were no alternatives to begin with, but some turned up in September of 1943. This had to do with experience at the Battle of Kursk. Priority work included modernization of the D-25T gun (which was already in progress), improvement of the muzzle velocity of the D-5T gun (first to 900 m/s then to 1000), and creation of a 100 mm tank and SPG gun. The TsAKB was tasked with the last item. Only work on the D-25T had any results. Production of a gun with a sliding breech began in February of 1944. In the meantime, results of penetration tests of German tanks became known, particularly of the Panther Ausf.D. The trials revealed an unpleasant fact. The upper front plate of this tank could not be penetrated by the 85 mm gun at a range of even 100 meters. Because of this, Malyshev gave orders to build a T-44 tank with an 1800 mm wide turret ring and a D-25T gun. A shortened fixed round was introduced to improve the rate of fire. As a result, the D-25T-44 gun had 2-4% lower muzzle velocity. The gun designed for the T-44 also had smaller and lighter recoil mechanisms.

The idea of using a D-25T gun with fixed ammunition on the T-44 was abandoned since the round was still too large and inconvenient. The ammunition capacity also became too small.

The third T-44 prototype quickly vanished from the list of projects. For one, the ammunition capacity of this T-44 was only 24 rounds, while the tank with the 85 mm D-5T-85 carried 54 rounds. Second, the gunnery trials brought only disappointment. The fixed ammunition was too long even for an 1800 mm wide turret ring. The rate of fire was just 3 RPM, giving almost no advantage. Later T-44 tanks were built with 85 mm guns and 1600 mm turret rings. The topic of a D-25T with fixed ammunition fell dormant, but not dead.

An attempt on a heavy chassis

The appearance of a 100 mm D-10T gun with a much higher rate of fire introduced some optimism regarding its implementation on the IS-2 tank. The idea of replacing the D-25 with the D-10 appeared several times, since calculations showed that the penetration of the two guns should be the same. However, the trials showed a different scenario. The D-10T penetrated the Panther from 1300-1400 meters, while the D-25T could do it from up to 2.5 kilometers. The idea of replacing the gun was dropped by the fall of 1944. The issue of the rate of fire of the D-25T remained unsolved. The gun was precise (on the level of the German 88 mm KwK 43 L/71) and had exceptionally high penetration, but the rate of fire was a big problem. The brass was not content to let this situation slide.

The ISU-122S was the only vehicle where the rate of fire issue was resolved via the installation of the D-25 gun that was supposed to use fixed ammunition.

On November 4th 1944 Stalin signed GKO decree #6868s "On improving usage of tanks and SPGs and on improving the quality of production". Item #5 of this decree required the NKV and NKB to prepare a proposal for introducing fixed ammunition for the D-25 by January 1st 1945. The same order was repeated in NKTP order #656s issued on November 6th 1944. The NKTP did not give up on increasing the rate of fire of 122 mm guns in the meantime. The ISU-122 was equipped with the D-25S gun. The result was the ISU-122S (Object 249) that was accepted into service on August 22nd 1944. The rate of fire with this gun increased to 5-6 RPM.

Blueprints of the fixed round. The AP round was 1.2 meters long and the HE round nearly 1.4 meters.

The issue of fixed ammunition was being worked on much more lazily. NII-24 began working on fixed ammunition with an AP shell towards the end of December of 1944 and finished it in early January of 1945. The minimum length of the round was 1203 mm. The letter indicated that this was not the limit, as the HE round would be 190 mm longer. The fixed ammunition would weigh 40 kg. The length of the casing was also increased by 60-65 mm. This ammunition only worked with a special variant of the D-25, which resulted in some disapproval from the GAU. They also shared some doubts regarding the reliability of holding these rounds.

Two results of trials from the NIBT Proving Grounds (1, 2). While the situation with the ISU-122S was evaluated a bit pessimistically, the IS-2 could indeed only be loaded by an acrobat.

The NIBT Proving Grounds was tasked with studying the viability of using fixed ammunition in the IS-2 and ISU-122S. Preliminary trials held on January 14th and 15th showed that it was possible to carry and fire fixed ammunition, but with some nuances. For example, the IS-2's gun could only be loaded at certain elevation angles. The rate of fire was not checked, but it was clearly not improved. In case of the ISU-122S, the situation was slightly better. The ammunition capacity even increased to 34 shots. Subsequent trials gave some unfortunate results. The length of the ammunition meant that it was almost impossible to load them in the IS-2 tank. The ammunition capacity dropped to 17 rounds. The situation with the ISU-122S was also described quite negatively, although the NKTP clearly bent the report in their favour. The ISU-122S had a lot fewer problems, even though the fixed ammunition was still a bad idea.

This image illustrates the chance of success at trying to fit fixed ammunition into the IS-3 tank.

While debates about the value of fixed ammunition continued, factory #9 produced two D-25 guns, one for tanks and one for SPGs. They could fire both fixed and separate ammunition. They were tested in Sverdlovsk with separate ammunition since no fixed ammunition was produced yet. Both guns were sent to the Kirov factory in May of 1945. The situation with tank production changed by then. Production of IS-3 tanks began in April and they completely replaced the IS-2 on the assembly lines later that summer. Naturally, the issue of using fixed ammunition in the IS-3 tank was raised. A simple truth was quickly discovered. It was not possible to fit the fixed ammunition in the IS-3's turret as it didn't have a large bustle. Based on how the ammunition racks were arranged, capacity would have been laughable. This is why Chelyabinsk factories refused to convert the IS-3 to take fixed ammunition. Attempts to force the Kirov factory to build the IS-3 with fixed ammunition were unsuccessful, and rightly so. This was a pointless waste of time.

There was no point in continuing the trials in November of 1945 since the ISU-122 was removed from service.

Unlike the IS-3, work on fixed ammunition for the ISU-122S continued. The Kirov factory reported in July that the fighting compartment would house 38 rounds of ammunition, but work dragged on. The ammunition was only ready in August of 1945. The ISU-122S was completed and sent to the NIBT Proving Grounds at Kubinka by the end of November of 1945. By that point the ISU-122S was no longer in production, and the result was predictable. Meanwhile, the requirement that a fixed round had to be shorter than a meter was voiced back in April of 1945. The artillery branch ignored this suggestion. They needed to have parts compatibility. 

The fix to the rate of fire problem on the T-10. A mechanical gun rammer was introduced to increase the rate of fire to acceptable levels.

The fixed ammunition was never accepted. To be fair, other nations had the same problems. The artillery branch wanted fixed ammunition, but if it turned out to be too big then separate loading was the only solution. The problem with reduced rate of fire was solved by using a mechanical loader. A rammer was supposed to be installed on the ZIS-6 107 mm gun, but work did not progress past general discussion. A rammer was also discussed for the D-25T back in 1943. Work only reached the practical stage on the Object 730. The mechanical gun rammer increased the aimed rate of fire to 3-4 RPM. The short fixed ammunition eventually appeared, but not in the USSR. The round for the 120 mm M256 gun is shorter than a meter and allows for convenient loading. Everyone runs into the same problems but finds different solutions for them. The USSR chose automatic loading, other nations chose smaller ammunition.


  1. I find it interesting Pasholok brings up the Rheinmetall/M256/NATO 120mm smoothbore ammo as a solution, when that was only possible due to propellants with a higher energy density and the development of semi-combustible cartridges.

    The equivalent western gun technologically to the D-25 family (and the M-62), the 120mm M58 on M103, also uses absolutely enormous split projectiles - which resulted in the M103 (and Conqueror) having comically large turrets with two loaders. Plans to use the gun with a single loader in a smaller turret ran into the IS-2/3/4 issue of low sustained rate of fire, which was considered unacceptable (otherwise it would have been the M60's gun).

    The ~8MJ energy of the D-25 is also substantially less energetic than the 11-11.5MJ of Rh-120 or 13-15MJ of M58. (Amusingly enough, the energetics of the M62 are almost identical to the Rh-120 and the 130mm C-70 to M58).

    1. I also have found thesis that Rh-120 and it's ammo (with good enought size and weight for one-piece cartridge) was designed due usage advanced technology from 70ties. Technology which can be used in 70ties in west Germany, probably can't be used in Soviet industry of 40ties. BTW- I have found "rule of thumb" that for human loader in a tank, one-piece cartridge can't be longer than 100cm and heavier than 25kg.

  2. Does the D-25S have any feature that makes loading quicker? Or it is just down to the ISU-122S having 2 loaders?

    1. I think it was just the second loader, although I'm not sure what he actually did since his title on the ISU-122 was breech operator, and the ISU-122S had a semiauto breech.