WW2 Soviet Heavy Tank Prototypes

KV-2 with 107 mm ZiS-6

Soviet Union (1941)
Gun Testbed – 1 Converted

In summer 1940, one of the ways through which Soviet officials wanted to improve the firepower of their heavy tanks was to mount an 107 mm gun in the KV-2. This idea was later dropped in favor of new, heavier tanks which were to directly mount the new 107 mm ZiS-6 gun. However, due to the slow progress of development, the KV-2 was brought back, and was used as a gun testbed for firing trials. Due to the start of the war, neither the ZiS-6 nor the heavy KV tanks designed to use it entered service.


Even before the KV-1 and KV-2 entered serial production, Soviet officials sought to improve their raw characteristics. One direction was to add even more armor, like on the T-150. Another was the improvement of the main armament. A document from June 1940 discussed the future of Soviet heavy tank developments. The second point mentioned the mounting of a high-power 107 mm gun in the KV-2. The gun was to have a muzzle velocity of 730-750 m/s and penetrate 100-110 mm of armor at 1,000 m. High rate of fire, high ammunition capacity, as well as both AP and HE shells were desired.

At the time, the KV-1 was still being produced with the underwhelming stopgap 76 mm L-11 gun, while the KV-2 was fitted with the 152 mm M-10T, which left much to desire in most areas. A 107 mm gun with such high velocity would have been formidable against any armored target at the time. It is perhaps this state of “overkill” which delayed the materialization of this project. Furthermore, the situation of the Soviet tank industry was not optimal. The creators of the KV tanks, the SKB-2 design bureau at LKZ (Leningrad Kirov Plant) were stressed with bug fixes for the KV-1 and were already tasked with the development of the T-150 and T-220 heavy tanks. Likewise, the creators of tank armaments, Plant No.92, were just beginning work on the much needed new tank guns, the 76 mm F-27 and 85 mm F-30.

New High-Power Gun

Plant No.172 had the 107 mm M-60 divisional anti-tank gun under development since late 1938, entering service in October 1940 under the designation M1940. However it was deemed too heavy to be mounted in a tank, thus a new gun was needed. It was likely this gun that was originally (in June 1940) meant to be mounted on KV-2. Based on V.G. Grabin’s memoirs (head of Plant No.92 design bureau), a KV-2 had been fitted with an 385 tonne/meter, 107 mm gun, likely an ancient M1910/1930 gun, to test the viability of mounting such a large gun.

The M1940 divisional gun. Although often assimilated with the ZiS-6, the latter was a completely new piece, specifically made for use on armored vehicles.
Source: Alf van Beem

A first mention of the 107 mm F-42 appeared on 16 December 1940, when Plant No.92 was already in the process of producing the first prototype. Curiously, they had requested a KV-1 tank to test fit the gun, and due to delays, allegedly fitted the 107 mm gun prototype into the T-28 medium tank instead. No testing was carried out due to lack of ammunition.

Here, it is worth mentioning the differences between the F-42 and M-60 anti-tank guns. The F-42 had a higher muzzle velocity of 800 m/s compared to the 730 m/s on the M-60. The barrel was also longer on the new gun, 5185 mm compared to 4605 mm on the M-60. The breech had also been altered. The M-60 used a clumsy door breech lock, while the F-42 used a vertical sliding breech lock. The shells of the F-42 weighed 18.8 kg.

Later documents reveal that this new 107 mm gun was originally to be designed at LKZ, with a deadline of November 1940, but the plant refused to do so. Only in January 1941 did the SKB-4, the artillery design bureau of LKZ, start work on their own 107 mm gun, the 412-2V, but it was canceled shortly thereafter due to the progres of the F-42.

Aside from the KV-2, in June 1940, the KV-220 was also proposed to be armed with an 107 mm gun. This idea came from the initiative of Grabin and J.Y. Kotin, head of the SKB-2 design bureau, despite the fact that the GABTU specifically requested the main armament of the KV-220 to be the F-30. By August, SKB-2 engineer G.N. Moskvin began working on this hypothetical weapon swap. He concluded that an 107 mm gun would have been too large to fit in the turret of the KV-220, especially considering how long the shells were, at 120 cm, making loading nearly impossible.

By December, it was clear that this gun was supposed to be the F-42, and Grabin still believed that his 107 mm gun would fit in the KV-220. To prove this, he traveled to Leningrad to convince Kotin. He then climbed into the turret (allegedly struggling to fit through the hatch) and attempted, and failed, to lift the 18.8 kg shell from the hull floor into the turret.

The story of the F-42 fitted in the KV-220 did not end here. On 19 February 1941, Marshall Kulik, who was overseeing progress at LKZ, submitted a report where he mentioned that the KV-220s second prototype was to be armed with the F-42 at Plant No.92, where the tank’s turret was waiting. Even more bizarre was that Kulik also mentioned that an F-42 gun was to be mounted on the KV-1 by May, but LKZ, understandably, outright refused to do so.

An abandoned KV-2 with the newer turret.
Source: Global Strategy

Tanks vs Guns

On 11 March 1941, the F-42, by this point renamed ZiS-6, re-entered the picture. Soviet intelligence services sent a report to the GABTU and Soviet leadership regarding the development of German tanks. In the section on heavy tanks, several projects were described, but perhaps the most alarming was that of a 90-tonne tank armed with a 105 mm gun.

This was not taken lightly, as neither the KV-1 nor the KV-220 in development were able to deal with such threats. Thus, SKB-2 was tasked by the GABTU with the design and development of a new heavy tank, specifically armed with the ZiS-6 gun, and weighing 72 tonnes. It was named KV-4 (Object 224).

A segment from Grabin’s memoirs showcases perfectly the situation of development at the time. On 4 April, Grabin was having a phone call with Stalin:

Stalin – “Hello, Comrade Grabin,- I want to consult with you. It is believed that a heavy tank which is armed with a low-power gun cannot meet the tasks of a heavy tank. Currently, the issue of re-equipping it is being considered: instead of the 76-mm gun, it is proposed to put a powerful 107 mm. I would like to know your point of view on this issue. It may be difficult for you to evaluate this proposal, since the heavy tank is armed with your 76-mm cannon.”

Grabin – “When our design bureau was issued tactical and technical requirements for a 76-mm gun for a heavy tank, we carefully studied the issues related to tanks and their weapons, and came to the conclusion that the 76-mm gun for a heavy tank is unpromising and does not even meet the requirements of today. We believed that a heavy tank should be armed with a more powerful gun, the projectile of which would penetrate the armor of its tank from a distance of 1000 m. The 76-mm cannon ordered to us was created and installed in the KV-1 tank.”

Stalin – “So, you’ve long had an opinion about the insufficient power of the 76 mm gun on heavy tanks?”

Grabin – “Yes, Comrade Stalin.”

Stalin – “It’s a pity that I didn’t know about it before. So, right now, our estimates do not contradict. Tell me, please, is it possible to put a powerful 107 mm gun in a heavy tank?”

Grabin – “Yes, Comrade Stalin.”

Stalin – “Are you sure that a powerful 107-mm gun can be put in a heavy tank?”

Grabin – “I am quite sure that a 107-mm powerful gun can be put in a heavy tank. But we consider the KV-2 tank unacceptable in terms of the design of the turret. The dimensions of the turret are large, and the shape of the turret is not optimal. Such dimensions for the 107-mm gun were not required.”

Stalin – “So you claim that a powerful 107-mm gun can be installed in a heavy tank?”

Grabin – “Yes, I am deeply convinced that a powerful 107-mm gun can be put in a heavy tank. If I understand you correctly, this gun should be higher in power than the 107 mm modernized?

Stalin – “You understood me correctly. The fact that you already have experience in installing a 107 mm cannon in a heavy tank is great. So, we will install a powerful 107-mm gun in a heavy tank?”

Grabin – “Yes, Comrade Stalin.”

Stalin – “This is very important, Comrade Grabin. As long as we do not arm a heavy tank with such a gun, we cannot be calm. The task must be solved as quickly as possible. This is required by the international situation. Tell me, could you be in Moscow tomorrow? We really need you here.

The following day, Grabin found himself at the meeting with the Secretary of the Central Committee of the All-Union Communist Party A.A. Zhdanov. Also present were the head of the GABTU ,Lt. Gen. Y.N. Fedorenko, as well as LKZ representatives J.Y. Kotin and I.M. Zaltsman, as well as V.G. Grabin and others.

Some of the individuals present at the Commissars office on the issue of mounting 107 mm guns on heavy tanks. From left to right: A.A. Zhdanov, V.G. Grabin, J.Y. Kotin, I.M. Zaltsman, Y.N. Fedorenko.

Zhdanov opened the meeting with a bold message:

“The party and the government attach great importance to the rearmament of a heavy tank, I ask you to approach with all seriousness the development of tactical and technical requirements and to determine the timing of the creation of a tank and gun. Deadlines should be minimal. Fascist Germany is walking around in the West. It is possible that in the near future it will attack us. We learned that the Germans are working on the creation of thick-armored tanks with powerful weapons. Our heavy tanks are poorly armed. […] “The draft decision needs to be prepared as soon as possible, therefore, you will have to work late, without leaving the Central Committee. You will eat here, we will provide it. A room is set aside for your work. I am at your disposal at any time…”

Things started off poorly. The LKZ engineers refused to mount a 107 mm gun on their tank. Most likely, this tank was the KV-220 and/or Object 222. This situation brought up a long-time argument between Kotin and Grabin, whether it was the mounting of the 107 mm gun in the KV-220, KV-1, or the mockup T-28 Grabin received instead of an actual tank for the testing of his F-39 gun. The situation was aggravated when Grabin, rather famously, said that the tank is just a chassis for the gun. Naturally, the tank designers did not take this lightly. After Zhdanov calmed down the 5 culprits, work commenced, but not smoothly. Grabin suggested that the new gun had to have 550 tonnes/meter, and with a longer barrel than the M-60. The tank designers objected, fearring that a too long gun would scrape up mud and lead to barrel damage.

The second day, most of the intricacies had been solved, with the exception of when everything should be ready. Once again, Zhdanov intervened:

Zhdanov – “When will the tank be ready?”

Kotin – “As soon as Grabin gives the gun, the tank will be ready,”

Zhdanov – “Comrade Grabin, when will you be able to give the gun?”

Grabin – “In forty-five days,”

Here Grabin recalled that everyone burst into tears, and that he was the only serious one in the room.

Zhdanov – “Comrade Grabin, we have gathered here to seriously resolve the issue, and you are joking.”

Grabin – “No, I’m not kidding, the term I mentioned is justified and quite serious.”

Zhdanov – “You continue to joke, go and think again.”

Grabin also highlights that this topic of 45 days was the subject of many jokes that he would hear often. With further consultation, the tank designers suggested that the number of days should be doubled, if not tripled. When it became clear that progress was not made, Zhdanov stepped in, again.

Zhdanov – “Well, Comrade Grabin, did you think through the deadline?”

Grabin – “Yes.”

Zhdanov – “Probably not forty-five days?”

Grabin – “Forty-five days, Comrade Zhdanov.”

Zhdanov – “And yet you are not serious. I think that the term should be significantly increased.”

Grabin – “Comrade Zhdanov, why does a short term cause homeric laughter and is considered frivolous, while a long term finds support and approval?”

Zhdanov – “We do not know of a single case where a new tank gun was created not only in forty-five, but even in ninety days. We believe that this will be a great opportunity for us to work together with our partners.”

Grabin – “I agree. Previously, it was not the case. Now it is. I ask you, Comrade Zhdanov, to write down in the draft decision: “The deadline for the production of a prototype tank and gun should be set forty-five days from the date of signing the decision.”

This was the deadline set in the contract. Grabin was set to leave back to Gorky even before the contract was fully signed. Before leaving, Zhdanov said that if Grabin was not able to meet the deadline, to just call him and tell Stalin and they would extend the contract. Grabin thanked him.

Naturally, much of the above should be taken with several grains of salt, as these clearly-one sided memoires were written many years after the fact. Nonetheless, they still provide valuable information on how Grabin viewed the ordeal.

On 7 April, the contract was signed, and further heavy tanks were ordered. The KV-3 (Object 223) and KV-5 (Object 225) were also meant to fit the same 107 mm gun. The KV-3 used an up-armored hull of the KV-220 and an enlarged turret, specifically made to fit the ZiS-6. The KV-5 was even larger than the KV-4, weighing 90 tonnes and with 170 mm of frontal armor.

Prototype of the ZiS-6. Note the coaxial DT machine gun.
Source: Yuri Pasholok
Collage of 2 photographs of the KV-3 mockup.
Source: Yuri Pasholok

The Gun is Ready, the Tanks are Not

Now that there were several vehicles in the works to mount the ZiS-6, work on it progressed. On the same day as signing the contract, 7 April, over at Plant No.92, a development program for the ZiS-6 was drawn up. Firing trials were to be undertaken on 15 May. Between April and May, ammunition production was set, and ballistic trials were laid out. However, several changes had to be made to the gun for use on tanks. Firstly, the shells were to be single piece, as opposed to 2-part. A gun rammer and bore evacuator were to be added to the gun for use on these tanks. On 14 May, the ZiS-6 was finished. Grabin was now able to boast that he made his gun one week faster, in just 38 days.

Naturally, work on the ZiS-6 was far more advanced compared to the heavy KV tanks and the gun had to be tested. The only vehicle readily available and able to mount the ZiS-6 was the KV-2. Thus, on 17 April, Fedorenko sent a letter to Zhdanov ordering that said gun was to be mounted on the KV-2 for trials. On 27 May, Plant No.92 shipped a ZiS-6 gun with serial number 2 (other sources claim 1) to LKZ for mounting on the KV-2, and arrived on 1 June. It was issued that the Izhora plant was to manufacture the gun mantlet and gun mount of the KV-3 and be mounted on a KV-2. Yet the Izhora plant was moving slowly, causing Marshall Kulik to intervene on 18 June, ordering the Izhora plant to deliver required parts and that testing shall commence with the ZiS-6 in a KV-2 with serial number B-9680.

While the KV-4 and KV-5 were long-term projects, the KV-3 was to act as a stopgap. On a contract signed 1 June, KV-3 was to enter production in August 1941, with 500 units to be built until the end of the year. Thus, Plant No.92 also had to deliver 500 guns before the end of the year. Grabin claimed that he began production of the guns far before it was ordered by the military. By the time the order of production was given, they had “dozens” of guns ready.

Front view of the KV-2 testbed with the new gun. Note the KV-3 mantlet.
Source: CAMO 38-11355-175

Design & KV-2

The KV-2 was born from the urgent need for a bunker buster to fight through the Finnish fortifications on the Karelian isthmus during the Soviet-Finnish war. However, due to the immediate need, a specially designed vehicle could not be finished in time. Thus, as a quick measure, an enlarged turret armed with an 152 mm M-10T gun was mounted on a prototype hull of the KV tank. The assault tank would prove slow and overweight, while the gun, despite its large size, had underwhelming performance against static defenses. The first 24 vehicles used a tall, complex turret, while later production models featured a more compact and efficient turret. In total, 204 such vehicles were built, but most were lost in the first stages of the fight against the Axis, most breaking down or being captured.

The first KV-2, called just KV big turret, or U-3.
Source: Juri-rust

The crew was the same as usual, comprising six men; commander, gunner, loader, and the loader assistant in the turret, and the driver and radio operator in the hull. As the hull was identical to that of the KV-1, it was powered by an 500 hp V-2K diesel engine, for a top speed between 30-35 km/h. It had two fuel tanks for a total of 600 liters.

As the tank was used as a testbed, much of the KV-2 tank remained untouched, with the main difference being that the gun mantlet had been changed to that of the KV-3 to accommodate the new gun. Due to the accelerated nature of the work, several details had not been worked out or converted. Firstly, the ammunition stowage racks had not been changed, so rate of fire was particularly low.

Interior view of the ZiS-6 in the KV-2 turret.
Side view of the KV-2 with the 107 mm gun.
Source: CAMO 38-11355-175
Rear view of the KV-2 at the Gorokhovets proving grounds. Note that from this angle, no changes can be seen.
Source: CAMO 38-11355-175


The tank arrived at the Gorokhovets proving grounds on 25 June, and the trials began. The ammunition used had additional explosive charges, increasing the muzzle velocity to 840 m/s. In total, 618 shots were fired, but the increased charges caused several issues. After 315 shots, the gun rammer broke. After 486 shots, the accuracy decreased considerably. The commission blamed these issues on barrel wear caused partly by the increased muzzle velocity of the shells. On the other hand, penetration was excellent. It penetrated 120 mm armor plates thick armor plates angled at 30° from 1,600 meters.

V.G. Grabin (white) at the trials of the KV-2, June 1941.
Still from the video above, showing the KV-2 firing the ZiS-6.

The End

The damaged gun was transported back to Gorky, where it was repaired. By 10 July, Plant No.92 had already completed four additional guns, as well as having 214 guns in various stages of production.

There was still no sign of any tanks for Grabin’s gun. The truth was that work on the KV-4 had completely stalled after May. Regarding the KV-3 and KV-5, work was continuing timidly. The KV-3 had a full scale mockup and most blueprints ready, while the KV-5 was still just entering the blueprint stage. Given the circumstances, the GABTU altered their request, asking for just 35 ZiS-6 guns, 5 in October, 10 in November and 20 in December.

The start of the war on 22 June significantly affected these projects, with LKZ having to ramp up KV-1 production, as well as focusing on repairs of tanks. The situation aggravated when German forces were approaching Leningrad in August, and thus the SKB-2 design bureau was evacuated to the ChTZ, in Chelyabinsk. While the KV-4 and KV-5 were outright canceled, plans of continued work on the KV-3 lingered. Eventually, by the end of 1941, it was obvious that nothing would come out of it either.

Realizing his mistake of prematurely starting production of the gun, in July, Grabin pitched the idea of fitting it on the KV-2 directly, but this went nowhere, as KV-2 production had already ended the previous month. In September, he proposed the ZiS-6A, a system similar to, if not directly for, the KV-4, with a coaxial 45 mm 20-K gun. This went nowhere as well.

Despite Grabin’s victory in his personal vendetta with the tank designers, his disappointment over the situation was considerable:

“The production of the ZiS-6 increased daily. But there was still no sign of the tanks for which the gun was intended. Even after the beginning of the war, the Kirov plant did not deliver a single tank. The lack of the new tank forced us to limit production and eventually cancel it. It is difficult and embarrassing to write about this, in those days when everything that could shoot, even museum pieces, were sent to the front, around 800 gun barrels had to be sent back to the melting furnace”

—V.G. Grabin

The story of the ZiS-6 does not end here. It was used to develop some even larger 107 mm guns, like the ZiS-24, but this gun also suffered a similar fate in spring 1942. More importantly however, the ZiS-6 was remembered in February 1943, when the Tiger appeared. Stalin called Grabin once again, who assured that he could produce the ZiS-6 in 15-20 days. However, nothing materialized.

The fate of KV-2 with serial number B-9680 is unknown, as it was last mentioned as being in Plant No.92s stock in September 1941.


The KV-2 107 mm testbed proved that there were several issues with the ZiS-6 gun, and Grabin’s confidence to start production prematurely would be a mistake. The gun still needed refinements, and eventually, the heavy KV tanks which were meant to mountmount it were canceled with the outbreak of the war.

KV-2 with 107 mm ZiS-6, ilustration by Pavel Alexe.
KV-2 with 107 mm ZiS-6 testbed Specifications
Dimensions (L-W-H) 6.95 (hull) x 3.32 x 3.25 (m)
Total weight, battle-ready >52 tonnes
Crew 6; commander, gunner, 2 loaders, driver, radio operator
Propulsion V-2K 500 hp diesel
Speed 30 km/h
Armament 107 mm ZiS-6 (F-42)
3x DT 7.62 mm machine guns
Armor Hull & turret: 70-75 mm
Roof & belly; 50 mm
No. Built 1 tested


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8 replies on “KV-2 with 107 mm ZiS-6”

Thought I’d just let you know there’s a typo in ‘The End’ section; “The story of the ZiS-6 does not end here. It was used to develop some even larger 107 mm guns, like the ZiS-24, but *****this gunthis gun*****”

Is the 90-tonne 105mm-armed German heavy tank mentioned in soviet reports the Panzer VII Lowe/VK70.01/VK. 72.02? It seems very similar to the Lowe, especially the 105mm cannon, but I understand that the Lowe may not have been in development yet.

Yes, sort of. The Soviets called it “Mark VII” in their report, matching with the Pz.Kpfw. VII index that the Löwe had, as well as the technical characteristics. The odd part is that German archive docs mention the tank earliest in November 1941, however its entirely possible that plans for the Löwe are much older.

Great article! I just wanted to point out there’s another typo:

Stalin called Grabin *once again once again,*

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