Monday 26 September 2022

Big Gun for a Big KV

ZIS-6 and other 100-107 mm tank guns developed in 1941

Vasiliy Gavrilovich Grabin is one of the best known Soviet gun designers. It just so happened that design bureaus under his direction created the most common tank and divisional guns used by the Red Army in the war. Grabin rose to fame as an artillery designer in the second half of the 1930s. He first created the F-22 and F-22 USV guns, then the F-32 and F-34. Far from all products developed at factory #92 went into mass production, but that was a typical scenario. The most important thing was that Grabin's designs were often better than its competitors. Factory #92's design bureau has no equal in 1940-1942. Their F-34 gun became the T-34's main weapon, and a variant of it called ZIS-5 became the main gun of the KV-1 in the fall of 1941. The Red Army's main towed gun was the ZIS-3, a grassroots initiative that could be used both as field artillery or an anti-tank gun. As an anti-tank gun, it was only beaten by another of Grabin's creations, the ZIS-2. The Central Artillery Design Bureau created on Grabin's initiative began to lose ground to factory #9's design bureau, but nevertheless the S-53/ZIS-S-53 was the T-34-85's main gun. The TsAKB was also responsible for the BS-3, another combination anti-tank and field gun. The BS-3, ZIS-S-53, and ZIS-3 still fight in distant corners of the world.

KV-2 with a ZIS-6 at the Gorohovets ANIOP, June 1941.

Factory #92 was working on a whole family of guns in early 1941, both towed and tank. They included the main character of this article: the 107 mm F-42 gun, later renamed to ZIS-6. Unlike most of factory #92's designs, this one was a grassroots initiative. However, a proposal to develop a 107 mm gun with the ballistics of the M-60 came about in June of 1940, and not from Grabin. In March of 1941 the ZIS-6 became relevant to a tank other than the KV-2. The gun became necessary for a whole number of new tanks that entered development that spring. Rumours and omissions are common when discussing the history of this gun, so let's fill in the gaps.

Not grassroots after all

There are many moments in the history of the F-42/ZIS-6 that might appear strange. One of them is the start of the gun's development. One might assume that Grabin decided he knew better than the army and created a gun that no one wanted. This was not the case. Grabin's design bureau was not the first where such a weapon was considered. The story behind this gun can be traced back to the summer of 1940. The GAU analyzed the armament of prospective tanks and SPGs, and decided that some of them have insufficiently powerful armament. For instance, the KV-1 had a 76 mm L-11 gun and then the F-32 with similar characteristics. A 76 mm gun with ballistics of the 3-K AA gun was proposed instead. The KV-2's gun was also insufficiently powerful. The M-10T was too weak to be a bunker buster and didn't have enough penetration to be a tank gun. Therefore, a proposal was made in June of 1940 to arm the KV-2 with a 107 mm gun with the ballistics of the M-60 divisional gun. This gun was the successor to the F-28 95 mm divisional gun that was developed in the late 1930s. The F-28 was built in February of 1940 and tested, but by the summer of 1940 it was clear that the M-60 was superior. The 95 mm gun also needed a new round. Introduction of a new caliber seemed risky.

The idea of arming the KV-2 tank with a gun that had the ballistics of the 107 mm M-60 was born in June of 1940. The Kirov factory was expected to do this work, but they refused.

Factory #92's design bureau did not yet have an order for a 107 mm tank gun. This work was proposed to Kirov factory's SKB-4 with a due date of November 1940. The factory declined this project. As you can see, the idea of a 107 mm tank gun was external. Factory #92 wasn't tasked with it since it was still working on the 95 mm F-39 tank gun. The only surviving information about this gun other than the fact of its existence is the installation layout, but it is a fact that the gun was worked on and built. It was tested in a T-28 tank and fired 45 shots. However, it was clear that the F-39 had no future since the F-28 gun was not being accepted into service. No one was about to introduce a new caliber just for tanks. Grabin urgently corrected the course of gun development in late 1940. Factory #92's design bureau was tasked with developing a 76 mm tank gun with 3-K AA gun ballistics and an 85 mm gun with 52-K AA gun ballistics. These guns were indexed F-27 and F-30 respectively. Prototypes were built in the fall of 1940 and tested in a T-28 tank. Towed guns with the same ballistics were also under development, the 76 mm F-26 on the F-22 USV divisional gun carriage and 85 mm F-7 (formerly F-28 UDL). The latter was a futile attempt at saving the F-28. Both guns were also built and tested in late 1940.

Status of work as of January 15th, 1941. An F-42 gun had already been installed in a T-28 tank, but not yet tested.

This is when a 107 mm gun appeared, and not just one. Grabin usually developed whole families of guns in one go, including towed and tank guns. Grabin also had not abandoned his attempts to save the F-28, offering a new gun on its carriage. There are some who consider this a waste of time, but they should not forget that factory #92's design bureau developed the 76 mm F-24 regimental gun. It was not put into production, but its carriage was used to build the F-31 57 mm anti-tank gun, better known as the ZIS-2. The M-60 was also not irreplaceable. Like the 152 mm M-10 that the M-60 was derived from (its carriage was close, but not identical) the gun was very heavy. 4 tons is a bit much for a divisional gun, especially one that is also developed for an anti-tank role. The F-28 weighed less than 2 tons, so the F-6 (107 mm divisional gun on the F-28 carriage) was much more than Grabin grasping at straws to save his brainchild. Excessive mass was one of the reasons that production of the M-10 and M-60 ended, so a lighter weapon would come in handy. However, no one considered that in 1941. On the other hand, the 107 mm tank gun was in a whole different league.

The F-42 first surfaced in correspondence on December 16th, 1940. A telegram from the factory reported that assembly of a prototype had already begun and asked for a KV-1 tank to install the gun into. Gorky must have been a city of optimists, as one can remember that the 85 mm F-30 required a new turret that resembled the KV-2 turret. Nevertheless, while the decision on whether or not to install the gun into a KV-1 was being made, factory #92 managed to fit it into a T-28. Trials had not been held by January 15th since no ammunition was available. It is not clear how Grabin's team fit the gun into a T-28's turret, but it was done nevertheless. The appearance of the F-42 gun (renamed to ZIS-6 in March of 1941) was a timely coincidence, but not factory #92's grassroots initiative. They simply carried out the work that Kirov factory refused to do. The F-42 also didn't have the same ballistics as the M-60. It used the same ammunition, but no more than that. The barrel length was different, 5185 mm vs the M-60's 4605. The muzzle velocity was also higher: 800 m/s vs the M-60's 730. The F-42 also had a sliding breech unlike the M-60's screw breech. Factory #92 essentially created a whole new gun, cleverly reusing old design concepts, particularly those from the F-30 and F-39.

Kirov factory attempted to return to the 107 mm gun program, except they were about half a year too late. 

Kirov factory's SKB-4 suddenly woke up after finding out about the F-42. They developed two systems: the 100 mm 412-1V and 107 mm 412-2V. The first gun was based on the B-24P naval gun produced at the Kirov factory (the factory was also supposed to build B-34 and B-54 guns). The issue was that there was no armour piercing shell available for these guns, but Kirov factory insisted that the 102 mm shell could be converted. The 412-2V would have had ballistics of the M-60 gun. No details on these guns had been found yet, but the GAU's response is known. It can be summarized as "where were you before?". The 100 mm caliber was refused outright, as there was no 100 mm AP shell. The 412-2V was accepted, but Kirov factory would have had to pay for a prototype themselves. Since the F-42 was already available, there was no need for another gun. Kirov factory's high caliber projects died in March of 1941 without being born.

A gun with no tank

At first work on the F-42 (already renamed to ZIS-6) was moving slowly. One problem was that there was no chassis for it. The T-150 tank that would be put into production under the index KV-3 after some improvements would have had either the F-34 or the F-27 (ZIS-5) gun. Due to issues with the T-220, work on the F-30 stalled. An even larger gun that needed an even larger turret was a difficult proposition.

Cutaway of the KV-3 tank. Note that the drawing shows fixed ammunition.

The situation changed in March of 1941. Soviet intelligence reported the appearance of German heavy tanks that weighed up to 90 tons. The ZIS-6 suddenly had a purpose. USSR Council of Commissars and Central Committee of the VKP(b) decree #827-345ss was issued on April 7th, 1941. It established technical characteristics for a new tank. It kept the name KV-3, but had a different blueprint number: 223. The tank would be armed with a ZIS-6 gun. The gun would take fixed ammunition. It was equipped with a mechanical gun rammer and fume extractor. There were some worries about fixed ammunition at factory #92, but Grabin insisted on following orders. This is why the KV-3 (223) blueprints show fixed ammunition. The tank would have carried 50 rounds.

Draft of a decree to put the ZIS-6 gun into production.

The KV-3 was not the end. Two more tanks that would use the ZIS-6 gun appeared. These were the KV-4 and KV-5 tanks that would have replaced the KV-3 in 1942. Trials would establish which one of them was better, but that was a far off future. The KV-3 was within immediate grasp. Production was scheduled to start in August of 1941 with 500 tanks delivered before the end of the year. Factory #92 was given an order for 500 guns that still had to be converted to take fixed ammunition, a gun rammer, and a fume extractor. Factory #92 would also have had to keep ahead of the tank factories in order to avoid delays in tank production.

A KV-2 tank with a hurriedly installed ZIS-6 gun before trials.

Even though this was a lot of work, factory #92 kept ahead of schedule. On the other hand, the assembly of the tank that the gun would be going into was not rushed. Even though the T-221 hull would be reused to build the KV-3 prototype, by mid-April it was clear that a new test platform would have to be found elsewhere. Head of the GABTU, Lieutenant General Ya.N. Fedorenko sent a letter to Zhdanov on April 17th indicating that a different tank would have to be used to test the ZIS-6. Ironically, the 107 mm gun that was initially going to be installed in a KV-2 tank ended up in one, but just for testing.

Marshal Kulik's personal involvement was required to get the gun installed.

Factory #92 sent a ZIS-6 gun with serial number 2 to the Kirov factory on May 27th, 1941. It was accompanied by mounting parts, a gun mantlet, and a DT machine gun. The gun arrived in Leningrad on June 1st, when it turned out that the system was not ready for installation. Kirov factory blamed the Izhora factory, as they were late on delivering the turret. There was a nuance here: the Izhora factory was building a KV-3 turret, but the decision to test in a KV-2 had already been made. Marshal Kulik had to get personally involved, signing an order to test the ZIS-6 gun in a KV-2 turret on June 18th, 1941.

Overall view of the gun.

A kick in the pants from the Deputy People's Commissar of Defense got the ball rolling. The gun was urgently installed in a KV-2 tank. This urgency had consequences. For one, no ammunition racks were installed. The rate of fire of the gun could not be measured. Trials of the fume extractor could also not be performed. The tank arrived at the Gorohovets ANIOP on June 25th. The report said that the gun installed was gun #1 with tube #8. The trials were also conducted with supercharged rounds, giving the gun a muzzle velocity of 840 m/s. Officially, the trials started on June 25th and ended on July 5th.

As requested, the gun had a rammer, fume extractor, and used fixed ammunition.

The gun made 618 shots. Traditionally, the description of the trials is boiled down to a note that the lifespan of the barrel was judged insufficient. Technically that is true, but other factors need to be considered. For one, the commission admitted that the cause of rapid wear was the supercharged round. Issues were only observed after the 486th shot. That is also when the drop in precision was observed. Until then, the precision was satisfactory. The penetration of the weapon was also sufficient. It could penetrate a 120 mm thick plate sloped at 30 degrees from a distance of 1600 meters, meaning that one KV-3 would be able to fight another. There were issues, for instance the rammer broke after 315 shots. However, it's not correct to say that the gun wasn't ready.

Trials of the gun. The tall man in a white jacket is Grabin.

The gun was returned to factory #92 after trials and a new series of test was conducted. Additional defects were discovered and had to be corrected. Mass production already began in June. There are a lot of arguments about this, but a report dated July 10th, 1941, shows that Grabin met his end of the bargain. 213 tubes were in various degrees of completion, at least 4 were finished. The plans had to be changed, since it was clear that the KV-3 would not be built in Leningrad. Production was moved to ChKZ along with the unfinished prototype. The production rate now looked a lot more humble. Factory #92 was now required to deliver only 35 guns: 5 in October, 10 in November, and 20 in December 1941. Grabin proposed installing the ZIS-6 gun into a KV-2 in July of 1941, but this was not done for the simple reason that production of these tanks ended.

Progress of building parts for the ZIS-6. Note that more of the work was completed than for the ZIS-2.

The last activity on the ZIS-6 project is dated September 1941. Factory #92's design bureau developed a variant of the gun named ZIS-6A. Many ascribe this system to the KV-7 assault tank, but they have nothing in common. This was an attempt at building a system like the one developed for the KV-4, with a coaxial 45 mm gun. Work stopped at the correspondence stage. Only factory #92 managed to meet the goals set by the next generation KV tank program. They were capable of producing a gun, but without a platform to put it into all their work was for nothing.

The same ZIS-6A that is so often mentioned alongside the KV-7. In reality, it has nothing to do with the assault tank.

Fortune turned against the ZIS-6. Production of the M-60 gun ended and other calibers gained priority: first the 85 mm (52-K AA gun) then 122 mm (A-19 corps gun). The third caliber was 100 mm. In the end, Soviet artillery returned to naval guns, settling for the fact that an AP shell would have to be urgently designed.

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