Video: Soviet Super-powered Tank Destroyers

 If the Maus or E-100 reached the battlefield during the Second World War, it would not be anywhere as invincible as it might have appeared. The Soviets had four prospective tank destroyers going through testing by the summer of 1945, each powerful enough to pierce its armour from a long range. Find out how and why in my latest video.

Heavy Tank Destroyers that Remained on Paper

The KV-14 (SU-152) SPG was accepted into service on February 14th, 1943. It was designed primarily to combat enemy fortifications, but its first use in combat pitted it against German tanks. Work on tank destroyers on the SU-152 chassis began in the spring of 1943. Even though none of these projects was implemented in metal, the work eventually led to the ISU-122.

OBM-43

"To the Chairman of the NKV Technical Council, comrade Satel

To the Chairman of the GAU Artillery Committee, Lieutenant-General comrade Hohlov

To the Head of the NKVD 4th Special Department, Commissar of State Security comrade Kravchenko

October 2nd, 1943

NKV special telegram #5014 sent on September 24th of this year tasked NKV OKB-172 and Molotov factory #172, as instructed by the People's Commissar of Armament and GAU Chief, to produce an SPG with a 122 mm gun that fired a 25 kg shell at 1000 m/s.

Object 257: The First IS-7

The IS-7 heavy tank is well known to armour enthusiasts. A combination of impressive armour, a powerful gun, and an engine that could propel the 70 ton tank at 60 kph made it the pinnacle of its class. At the same time, no less than 7 vehicles existed under the IS-7 index, and three of them were called "Object 260". The creation of this vehicle is shrouded in mystery, partially due to the conditions of secrecy created at the factory #100 design bureau. Thanks to recently discovered archive materials, we can now see how the IS-7 (back then, still Object 257) developed in its early stages.

Experimental Guns

"Decree of the State Committee of Defense #4851ss
December 27th, 1943
Moscow, Kremlin

On the design, production, and trials of experimental guns for tanks and SPGs.

The State Committee of Defense decrees that:

1. On the 85 mm tank gun: GAU chief comrade Yakovlev and Armoured Forces commander comrade Fedorenko must perform trials of the 85 mm S-53 and S-50 guns installed in T-34 tanks (S-53 in a T-34 with a stock 1420 mm turret ring, S-50 in a tank with a widened turret ring) and the 85 mm LB-1 gun designed by factory #92 in a T-34 with a widened turret ring before December 30th, 1943.
   Report to GOKO on results by January 2nd, 1944.
2. On the 100 mm tank gun (comrade Grabin):
1. NKV (comrade Ustinov) and TsAKB (comrade Grabin) must provide the S-34 100 mm tank gun for trials by January 1st, 1944.
   GAU chief (comrade Yakovlev) and Armoured Forces commander (comrade Fedorenko) must perform trials by January 5th, 1944.
   Report to GOKO on results by January 7th, 1944.
2. NKV (comrade Ustinov) and factory #92 director comrade Yelyan must produce two S-34 100 mm guns according to blueprints from TsAKB, one for the IS tank and one for an SPG on the T-34 chassis to factory #100 by January 25th, 1944.
3. NKTP (comrade Malyshev) and factory directors of Kirov factory (comrade Zaltsmann), factory #100 (comrade Kotin) and Uralmash (comrade Muzrukov) must design and produce an IS tank and an SPG on the T-34 chassis with 100 mm guns.
   Perform factory trials of the IS tank and SPG with 100 mm guns by February 20th and provide the tank and SPG for proving grounds trials by February 25th.
4. GAU (comrade Yakovlev) and the Armoured Forces commander (comrade Fedorenko), in cooperation with NKTP and NKV, must, within five days, perform trials of the IS tank and SPG with 100 mm guns and deliver a conclusion to GOKO by March 5th, 1944.
5. TsAKB (comrade Grabin) must include the possibility to install an 85 mm barrel into the S-34 mount to achieve a muzzle velocity of 1000-1100 m/s and begin the production of this gun with the aim of producing an experimental prototype for government trials by April 1st, 1944.
6. People's Commissariat of Ammunition (comrade Vannikov) must produce ammunition (casing, propellant, AP and HE shells) for the 85 mm TsAKB gun with a muzzle velocity of 1000-1100 m/s and provide 1000 proof rounds as well as 250 each HE and AP rounds.
3. On the 130 mm tank gun (comrade Grabin):
1. NKV (comrade Ustinov) and TsAKB (comrade Grabin) must design by January 5th 1944 and produce by March 1st 1944 an experimental prototype of a 130 mm gun, with the ballistics of the 130 mm B-13 naval gun, to be used in an SPG on the IS chassis.
2. Director of factory 232, comrade Zakharyin, must produce a barrel with breech and other components, as well as cast and forged components, and deliver them to TsAKB by the aforementioned date.
3. NKV (comrade Ustinov) and TsAKB (comrade Grabin) must send one 130 mm SPG gun to factory #100 by March 10th, 1944.
4. NKTP (comrade Malyshev) and directors of factory #100 and Kirov factory must design an SPG on the IS tank chassis to use a 130 mm gun.
   Begin factory trials of the 130 mm SPG by April 1st, 1944, and provide it for proving grounds trials by April 5th, 1944.
5. GAU (comrade Yakovlev) and the Armoured Forces commander (comrade Fedorenko), in cooperation with the NKTP and NKV, must complete proving grounds trials in five days' time and report to GOKO with their conclusions by April 10th.
6. People's Commissar of Ammunition (comrade Vannikov) must produce 500 each of HE shells for the 130 mm SPG, AP and HE shells for the 122 mm gun with a muzzle velocity of 1000 m/s, and deliver them by March 20th, 1944 for trials.
4. On the increased muzzle velocity of the 122 mm tank gun:
1. NKV (comrade Ustinov), TsAKB (comrade Grabin), OKB-172 (comrade Ivanov) and factory #172 director (comrade Bykhosvkiy) must design and produce two prototypes of a 122 mm gun with a muzzle velocity of at least 1000 m/s using a 25 kilogram shell (one designed by TsAKB and one by OKB-172) to be used in an IS SPG.
   Submit finished prototypes to factory #100 for installation in vehicles by March 10th, 1944.
2. NKTP (comrade Malyshev), Kirov factory director (comrade Zaltsmann) and factory #100 director (comrade Kotin), in cooperation with TsAKB and OKB-172, must design IS SPGs with TsAKB and OKB-172 122 mm guns.
   Finished SPGs must be submitted for proving grounds trials by April 1st, 1944.
3. GAU (comrade Yakovlev) and the Armoured Forces commander (comrade Fedorenko), in cooperation with the NKTP and NKV, must complete proving grounds trials of the SPGs within 10 days and submit their conclusions to GOKO by April 15th, 1944.

Chair of the Committee of Defense, I. Stalin."

RGASPI 644-2-254

Experimental SPG Work, January 1944

"To the member of the State Committee of Defense, comrade L.P. Beria

On the letter from the People's Commissar of Tank Production, I report:

1. GAU supports the NKTP's request to cease producing the 130 mm gun, indicated in the GOKO degree issued on December 27th, 1943.
   GAU does not take part in the arguments between NKTP and TsAKB on the possibility of installing a 130 mm gun into the SPG on the IS chassis, knowing that if the organizations are willing to make concessions, a decision that satisfies both sides can always be reached.
   GAU considers it necessary to cancel the 130 mm SPG, as it requires a more reasonable and mass produceable artillery system for tanks and SPGs, which is possible to build in 1944.
2. Our current SPGs with 122 mm and 152 mm guns model 1937/43, with ballistics of corps level guns A-19 and ML-20 are powerful weapons against German heavy tanks and SPGs, but it is necessary to further increase the firepower of these weapons.
   These requirements are confirmed by developments of the arms race on the battlefield and possible technical progress by our enemy.
   Considering this scenario, OKB-172 has designed a high power 122 mm SPG gun according to GAU specifications, indexed OBM-50. The technical project has been reviewed and approved by Artkom on December 18th, 1943. An experimental prototype has been ordered, and should be completed by factory #172 in February. The production of the 122 mm OBM-50 gun is confirmed by GOKO decree #4851ss issued on December 27th, 1943. In order to perform factory and proving grounds trials of the 122 mm high power gun, we need you to order the People's Commissariat of Ammunition to produce new tube nitroglycerine propellant.
   GAU considers this gun equivalent in power to the 130 mm gun proposed by comrade Grabin, but the 122 mm high power gun has many advantages over the 130 mm gun.
3. The 152 mm OBM-43 gun is a completely different class of weapon than comrade Grabin's 130 mm gun, and can't be compared.
   The OBM-43 has been built according to GAU specifications as a towed gun. It is possible to install it in an IS SPG using the oscillating part and mount from the 152 mm model 1937/43 SPG gun (SU-152), which was ordered by comrade Malyshev and carried out by factory #100 and OKB-172 engineers.
   This 152 mm gun is not fully developed and is only an intermediate step, and its trials are of little interest. GAU does not oppose the transfer of the 152 mm OBM-43 gun from the NKV to factory #100 for mounting in an IS SPG to carry out trials at the Gorohovets proving grounds with assistance from the GBTU in order to test several design solutions and use cases.
4. The installation of a special 152 mm SPG gun with the ballistics of the OBM-43 in an SPG has been foreseen by GAU. The project for such a gun, indexed OBM-53 has been reviewed and approved by Artkom on December 18th, 1943, at the same time as the OBM-50 project.
   No prototype is being produced, as the NKV has not issued the appropriate orders to factory #172. I ask you to order the People's Commissar of Armament, comrade Ustinov, to produce an experimental OBM-53 152 mm SPG gun by February 25th.
   GAU considers the 152 mm OBM-53 gun to be a very powerful weapon, and, due to design specifics and high power, it cannot be a mass produced tank escort weapon. It must be built in limited numbers to achieve objectives set by the Supreme Command.
5. GAU supports the NKTP proposal to test a 100 mm gun designed by factory #9. At the same time, GAU considers it necessary to test the 100 mm gun designed by TsAKB.
6. Along with the 100 mm gun, GAU considers it necessary to test an 85 mm gun with the muzzle velocity of 1000-1100 m/s.
   This gun has nearly the same firepower and penetration as the 100 mm gun, but the weight and size give it an advantage over the 100 mm gun, both of the gun and the shell.
   Additionally, the production of an 85 mm gun and ammunition for it would not pose any difficulty, as the design of the new gun and its shells is an organic progression from the existing items produced at NKV and NKB factories. Savings in metal (approximately 30% for the gun and 40% for its shells) are also a benefit.
7. When installing powerful guns with barrels from 5600 mm (85 and 100 mm guns) to 7000-7500 mm (122 and 152 mm guns) in length, the barrel will exceed the front of the SPG by at least 3500 and up to 5000 mm (with the 152 mm gun). Because of this, when moving off-road, it will hit objects and terrain, which will cause damage to the gun and knock it out during battlefield maneuvers.
   In January of this year, the commander of artillery of the 3rd Ukrainian Front sent a report to GAU, regarding the SU-85 SPG. In this SPG, the barrel exceeds the length of the SPG hull by about 2300 mm. The report says that when going over obstacles, there were cases of sand getting into the barrel, and strong enough hits against the ground that the normally reliable sliding shaft of the elevation mechanism bends, and the SPG cannot be used. The possibility of this happening with high power SPGs is much greater.
   Measures suggested by the NKTP (pushing the barrel back by 800 mm, 10 degrees of gun elevation) help only on the march, but not in combat, where damage is being done to these guns.
   Based on the above, GAU considers it necessary to, in parallel with the design of new powerful guns, to design new SPGs with rear fighting compartments (like the SU-76).
   This will preserve the high combat maneuverability of SPGs and allow the increase of their front armour.
8. The armament of a self propelled gun must not only be examined from the point of view of increasing the firepower of tank units, but from the point of view of increasing the firepower of infantry units, maneuverability and firepower of special tank destroyer units, and creation of special weapons for the Supreme Command Reserve.
   GAU considers it necessary to have the following weapons in self propelled artillery:
1. Infantry support SPGs
1. 76 mm ZiS-3 gun (F-34) on a light SPG chassis made from T-70 components.
2. 122 mm D-5 howitzer or 152 mm D-15 howitzer on a medium SPG chassis made from T-34 components.
2. Tank escort SPGs
1. 85 mm D-5 gun (with AA gun ballistics) on a medium SPG chassis made from T-34 components, already in production.
2. 85 mm high power gun or 100 mm gun on a medium SPG chassis made from T-34 components, planned for production in 1944.
3. 122 mm D-25 gun or 122 mm A-19S gun or 152 mm mod. 1937/43 gun, already in production, installed on a chassis made from IS tank components.
4. High power 122 mm OBM-50 gun on an SPG chassis made from IS tank parts, planned for production in 1944.
3. Supreme Command reserve SPGs
1. 152 mm OBM-53 gun installed on an SPG chassis made from IS tank parts, planned for production in 1944.
2. 203 mm howitzer on a half-covered chassis made from IS tank parts.
4. Tank artillery
1. Medium tanks:
1. 85 mm D-5 gun (AA gun ballistics) or 85 mm high power gun.
2. Heavy tanks:
1. 122 mm D-25 gun.
2. 100 mm gun or 85 mm high power gun.
9. GAU considers existing artillery systems in tanks and SPGs incomplete. The correlation between quality and quantity of SPGs still needs refinement. Attempts to correct this balance, as experience shows, appear as individual and sometimes poorly thought out proposals from many design organizations. The demands of the front and capabilities of production are not considered.
   When these individual projects are built, antagonistic relationships form between organizations. This can cause unrefined prototypes to be pushed into production prematurely or the opposite, a delay before introducing this weapon into the Red Army.
   In order to bring order to the design of tank and SPG armament, GAU deems it necessary to hold a meeting of representatives of the General Staff, GBTU, GAU, NKV, NKTP, NKB, and design organizations in order to refine tank and SPG artillery systems.

Red Army GAU Chief, Colonel-General of Artillery, Yakovlev

January 24th, 1944"

CAMD RF 81-12038-303

Soviet Heavy Tank Destroyers

I already covered light and medium SPGs, so let's move on to heavy SPGs in CAMD RF 38-11369-1.

"Experimental work on heavy SPGs.

The creation of heavy artillery SPGs follows the goal of increasing the firepower of tanks. Since placing 152 mm guns in the turrets of existing tanks was not possible, it was deemed reasonable to place these artillery systems in immobile fighting compartments. In order to preserve the production numbers of SPGs at tank factories, the overall component layout was preserved. As experience showed, unification of tanks and SPGs was very effective. It was possible to set up mass production in very short amounts of time with minimal decrease in production numbers. In order to fill the Red Army's need for new heavy SPGs, the following work was done.

1. On the ISU-152 SPG.

According to GOKO decree, in December-January of 1942-1943, the SU-152 was designed and built by the Kirov factory. The SU-152 SPG consists of a 152 mm gun-howitzer on a KV-1S chassis. The SPG was tested in January-February of 1943. GOKO decree #2883 on February 14th, 1943 adopted the vehicle into the Red Army.

Mass production SU-152 produced by the Kirov factory

Experimental ISU-152 SPG produced by factory #100

According to a State Committee of Defense decree, factory #100 designed an experimental ISU-152 SPG in June-September of 1943. The ISU-152 consisted of a 152 mm ML-20 gun on an IS tank chassis. In October of 1943, the SPG underwent trials at NIBT and GANIOP proving grounds over 430 km and 303 shots.

1. Combat mass: 45.5-46 tons
2. Crew: 5
3. Front armour: 60-100 mm
4. Side and rear armour: 60 mm
5. Armament: 152 mm ML-20 gun-howitzer
6. Muzzle velocity: 600 m/s
7. Horizontal traverse: 10 degrees
8. Vertical traverse: -3 to +20 degrees
9. 152 mm shells: 20
10. Average highway speed: 23.4 kph
11. Maximum speed: 37.5 kph
12. Average off-road speed: 18.2 kph
13. Engine: V2-IS
14. Maximum engine power at 2000 RPM: 520 hp
15. Fuel type: diesel
16. Fuel capacity: 500 L

The experimental ISU-152 SPG passed trials and was recommended for adoption by the Red Army. The ISU-152 was adopted into the Red Army by GOKO decree 4504ss on November 6th, 1943, and is still produced at the Kirov factory.

Compared to the SU-152, the ISU-152 has stronger armour and a more robust transmission, engine, and suspension design. The armament was unchanged, as the ML-20 was powerful enough for any application. The ammunition rack, sights, observation devices, among other components, were also improved.

Engineer-Major comrade Rabtsev worked admirably in designing the ISU-152 SPG. As a part of a commission led by Engineer-Colonel comrade Demyanenko, comrade Rabtsev corrected several defects in the ML-20 gun mount.

2. On the ISU-122 and ISU-122S SPGs.

An experimental prototype of a heavy SPG with a 122 mm A-19 gun instead of the ML-20 was built by factory #100 in December of 1943. The work was conducted in agreement with USA GBTU to increase the anti-tank power of the SPG. This was achieved by replacing the 152 mm gun barrel with the 122 mm A-19 barrel. The gun mantlet and ammunition rack were changed, without alterations to any other components of the production SPG. 

With the installation of a 122 mm gun, the following was achieved:

1. High penetration. 122 mm shells with a muzzle velocity of 800 m/s penetrated thick armour, which was necessary in battle with German tanks and SPGs.
2. High rate of fire and precision. The rate of fire of the A-19 was twice as high as the rate of fire of the ML-20.
3. Increase of the ammunition rack from 20 shells to 30 shells.
4. Reduction of the combat weight due to a lighter gun system.

Trials of the ISU-122 were conducted at the Gorohovets proving grounds from December 24th to December 31st, 1943 over 463 shots by a commission led by Major-General of the tank engineering service comrade Alymov. The SPG passed trials. Based on this, the ISU-122 was recommended for service in the Red Army. By GOKO decree #5378 on March 12th, 1944, it was put into mass production at the Kirov factory.

As a result of the 122 mm D-25 gun with identical ballistics entering production at factory #100, the factory was tasked to design an SPG that uses the D-25 gun. In April 1944, the ISU-122S SPG was built. The ISU-122S is a further modernization of the ISU-122 SPG that increases rate of fire. The D-25 is designed as a tank gun, and its recoil mechanisms and breech are more compact than on the A-19. As a result of this, the crew was more comfortably placed in the fighting compartment. The shorter recoil and sliding breech (the A-19 had a screw breech) increased the rte of fire (from 2.5 RPM on the A-19 to 6 on the D-25). The unification of tank and SPG guns was also important.

The experimental ISU-122S prototype was tested at the Gorohovets proving grounds from June 2nd to June 7th, 1944, by a commission headed by Engineer-Colonel comrade Kupriyanov over 446 shots and 192 km. Tests were conducted carefully and with a high level of mastery. Due to superior qualities of the ISU-122S SPG over the ISU-122, it was recommended for adoption into the Red Army. GOKO decree #6430 on August 22nd, 1944 adopted the ISU-122S for service. The ISU-122S remains in production at the Kirov factory to this day. It is important to point out that both the ISU-122 and ISU-122S are still in production. This is due to the fact that D-25 production is insufficient to cover demand.

Mass production ISU-122 with 122 mm A-19 gun built at factory #100

1. Combat mass: 45.5-46 tons
2. Crew: 5
3. Front armour: 60-100 mm
4. Side and rear armour: 60 mm
5. Armament: 122 mm A-19 gun
6. Muzzle velocity: 781 m/s
7. Horizontal traverse: 10 degrees
8. Vertical traverse: -3 to +20 degrees
9. 122 mm shells: 30
10. Maximum speed: 37.5 kph
11. Average highway speed: 23.5 kph
12. Average off-road speed: 18.2 kph
13. Engine: V2-IS
14. Maximum engine power at 2000 RPM: 520 hp
15. Fuel type: diesel
16. Fuel capacity: 500 L

1. Combat mass: 44.5-45 tons
2. Crew: 5
3. Front armour: 60-100 mm
4. Side and rear armour: 60 mm
5. Armament: 122 mm D-25S gun
6. Muzzle velocity: 781 m/s
7. Horizontal traverse: 11 degrees
8. Vertical traverse: -2 to +20 degrees
9. 122 mm shells: 30
10. Maximum speed: 37.5 kph
11. Average highway speed: 25 kph
12. Average off-road speed: 18.1 kph
13. Engine: V2-IS
14. Maximum engine power at 2000 RPM: 520 hp
15. Fuel type: diesel
16. Fuel capacity: 500 L

3. On the ISU-122BM and ISU-130 SPGs.

In order to further improve the armament of heavy SPGs, the government decided to produce new 122 mm guns with a muzzle velocity of 1100 m/s and 130 mm guns with a muzzle velocity of 900 m/s. According to GOKO decree #4851 from December 27th, 1943, factory #100 produced three experimental prototypes: two ISU-122BMs and one ISU-130, equipping production SPGs with these guns. 

The ISU-122BM SPGs were built wit a 122 mm BL-9 gun designed by OKB-172, and one with a 122 mm S-26-1 gun designed by TsAKB. The BL-9 and S-26-1 have identical ballistics and differ in component design. The muzzle velocity of these guns is over 1000 m/s, and 200 m/s higher than the D-25. The increase of muzzle velocity increased the penetrative power of the guns. 

The experimental SPGs were late due to delays in producing the guns. The ISU-122BM with a BL-9 gun was delivered for trials on July 9th, 1944. The second, with the S-26-1 gun, was delivered to trials on November 15th, 1944, instead of April 1st, 1944, the term stated in the GOKO decree.

The ISU-130 was produced and delivered for trials on November 25th, 1944, instead of April 5th, as per decree. The prototype used a 130 mm S-26 gun designed by the TsAKB, based on the B-13 naval gun.

Trials of the three SPGs were split into three stages.

• First stage: preliminary artillery trials at GANIOP in Leningrad.
• Second stage: proving grounds gunnery and mobility trials. Gunnery trials are to be held after the guns pass preliminary trials.

Trials results:

The ISU-122BM with a 122 mm BL-9 gun designed by OKB-172 failed trials due to insufficient robustness of the barrel (August, 1944). A decision was made to improve the barrel and test the gun again. A new barrel was ready by February of 194. The delay with the new barrel and the S-26-1 barrel was due to the fact that NKV only produced one of each barrel. After their breakage, a lot of time was needed to make new ones.

Despite the demands of the Military Council of the Armoured an Motorized Forces of the Red Army to produce backup barrels, NKV did not do so, and did not take into practice producing experimental guns with large muzzle velocities. Delays were also caused by factories #172 and #232, which delivered poor quality barrels.

The BL-9 passed preliminary trials in 1945, from April 5th to May 30th. The ISU-122BM is currently undergoing trials in parallel with the other ISU-122BM SPG.

The ISU-122BM SPG with the S-26-1 gun failed November trials. Its barrel was also insufficiently robust. Correction of the gun took a long time, for the same reasons as above. Only by June 1945 was a new barrel installed on the SPG to make it ready to resume trials. Which of these SPGs is superior and which will be mass produced will be discovered in the near future.

With an SPG armed with a similar gun, the Red Army will gain a new powerful method to combat even heavier enemy tanks and SPGs.

Experimental prototype of the ISU-122BM SPG built at factory #100

1. Combat mass: ~47 tons
2. Crew: 5
3. Front armour: 60-100 mm
4. Side and rear armour: 60 mm
5. Armament: 122 mm BL-9 or S-26-1 gun
6. Muzzle velocity: 1000 m/s
7. Horizontal traverse: 10 degrees
8. Vertical traverse: -1 to +18 degrees 40 minutes
9. 122 mm shells: 20
10. Maximum speed: ~37.5 kph
11. Average highway speed: 23.4 kph
12. Average off-road speed: 18.2 kph
13. Engine: V2-IS
14. Maximum engine power at 2000 RPM: 520 hp
15. Fuel type: diesel
16. Fuel capacity: 500 L

The ISU-130 SPG underwent artillery trials at the Leningrad proving grounds twice, once from November 24th to December 8th, 1944, and once from December 14th to December 31st, 1944. 663 shots were fired in total (1st stage: 359 shells, 2nd stage: 304 shells). After defects found in the second stage, the ISU-130 was returned to TsAKB to be corrected for further trials.

Despite numerous demands from GBTU, the correction took until May of 1945. Currently (June 1945) it is undergoing mobility and additional gunnery trials. The ISU-130's gun is a naval caliber gun, and has never before been used by the Red Army. However, considering the positive qualities of the 130 mm gun, it may be worthwhile to introduce a new shell caliber.

Experimental prototype of the ISU-130 with a 130 mm S-26 gun

1. Combat mass: ~47 tons
2. Crew: 5
3. Front armour: 60-100 mm
4. Side and rear armour: 60 mm
5. Armament: 130 mm S-26
6. Muzzle velocity: 900 m/s
7. Horizontal traverse: 7 degrees 50 minutes
8. Vertical traverse: +16 degrees 30 minutes
9. 130 mm shells: 20
10. Maximum speed: 37.5 kph
11. Average highway speed: 23.4 kph
12. Average off-road speed: 18.2 kph
13. Engine: V2-IS
14. Maximum engine power at 2000 RPM: 520 hp
15. Fuel type: diesel
16. Fuel capacity: 500 L

4. On the ISU-152BM SPG.

The experimental ISU-152BM prototype was produced by factory #100 on its own initiative in April of 1944. This prototype used the BL-8 152 mm gun at first, then the BL-10 gun, produced by factory #172. These guns were installed in a production ISU SPG.

The BL-8 and BL-10 guns have the same muzzle velocity of about 850 m/s and identical ballistics. They differ in the breech type. The BL-8 has a screw breech, while the BL-10 has a semi-automatic sliding breech. 

In July of 1944, the BL-8 gun was tested on the Leningrad artillery proving grounds over the span of 501 shots. Trials showed flaws in the gun's design. As a result of July trials, a new gun was built, indexed BL-10. In the end of August of 1944, the ISU-152BM SPG with a BL-10 gun was delivered to the Leningrad artillery proving grounds for trials. However, trials were delayed due to a delay in procuring the new type of AP shell.

In December of 1944, the BL-10 was trialled, but failed trials due to insufficient robustness of the barrel, and was sent back for corrections. Production of a new barrel was delayed due to defects at factory #172. A new gun barrel was only built and produced by July of 1945.

With the successful correction of this gun's defects, the Red Army will receive a new, more powerful, artillery system that can defeat any known or potentially built tanks and SPGs of the enemy. Aside from its AP performance, the gun has good HE performance against enemy artillery and other targets. 

The SPG may be further improved by reducing the overhang of the barrel by moving the fighting compartment backward and widening the fighting compartment.

Experimental prototype of the ISU-152BM SPG built at factory #100

1. Combat mass: ~47 tons
2. Crew: 5
3. Front armour: 60-100 mm
4. Side and rear armour: 60 mm
5. Armament: 152 mm BL-10
6. Muzzle velocity: 850 m/s
7. Horizontal traverse: 8 degrees
8. Vertical traverse: -3 degrees to +18 degrees
9. 152 mm shells: 21
10. Maximum speed: 30-34 kph
11. Average highway speed: 23-25 kph
12. Average off-road speed: 18 kph
13. Engine: V2-IS
14. Maximum engine power at 2000 RPM: 520 hp
15. Fuel capacity: 500 L

Experimental prototype of the IS-3 based heavy SPG produced at the Kirov factory

1. Combat mass: ~47 tons
2. Crew: 5
3. Front armour: 120 mm
4. Side armour: 90 mm
5. Rear armour: 60 mm
6. Armament: Modernized 152 mm ML-20SM gun-howitzer
7. Horizontal traverse: 10 degrees
8. Vertical traverse: -2 degrees to +18 degrees
9. 152 mm shells: 20
10. Maximum speed (calculated): 37 kph
11. Engine: V2-IISI
12. Maximum engine power: 520 hp
13. Fuel type: diesel
14. Fuel capacity: 650 L"

Penetration, Part 4

This article covers a number of guns from 1945. Some of them you have seen before, some of them are new.

CAMD RF 38-11369-696

The header row lists the guns the document covers: 85 mm D5-85-BM (factory #9), 85 mm D10-85, 85 mm S-34-IV (TsAKB), 122 mm S-34-II (TsAKB), 122 mm D-30 (factory #9), 122 mm BL-9 (OKB-172), 122 mm S-26-I (TsAKB), 130 mm S-26 (TsAKB), 152 mm BL-10 (OKB-172), and the 122 mm BL-13-1 (OKB-172). 

The second row shows the vehicle class the gun would have been mounted on. The first three 85 mm guns are for medium SPGs (presumably, it just says SU). The next two 122 mm guns are for the Object 701 (IS-4) and IS-6 respectively. The next 4 guns of various calibers are for heavy SPGs (same, but ISU). The last gun is marked for the IS-6, but crossed out. 

The next row is for the muzzle velocity of the shell, AP (top) and HE (bottom). The row after is for maximum ranges. The range of most guns in the ISU is limited by the gun's elevation, rather than the propellant.

After that is what we came here for, the penetration values. The numbers for the D-30 are similar to that of the D-25 we have seen in previous tables. The penetration of the BL-10 is a little lower than the value in the first article of this series; it has been reduced by 4 mm at 100 meters, and also drops off faster. 

The D5-85-BM, on the other hand, received a marginal improvement. The D10-85 blows it out of water with penetration surpassing that of the slower 122 mm guns. The S-34-IV performs similarly. 

The line after that is the rate of fire. After that, ammunition capacity. After that, crew members in the gun's vehicle. 

Soviet Very High Power Guns: BL-9

I have previously written about the BL-8 152 mm Very High Power gun, and briefly mentioned the BL-9 Very High Power gun. Here is how both of these projects started. Surprisingly enough, they were originally meant not for the ISU-152, but for the SU-152. CAMD RF 81-12063-1 tells the interesting story.

"The special NKV telegram #5014 sent on September 24th, 1943, tasked OKB-172 and the #172 Molotov factory to develop a 122 mm self propelled gun, capable of reaching 1000 m/s muzzle velocity with a 25 kg shell.
Giving the highest priority to this project, OKB-172 completed the draft ahead of schedule, and presents it for your approval. As a result of performing the draft work, we have discovered the following:

1. The task of creating an SPG with the muzzle velocity of 1000 m/s can be accomplished by installing a new barrel on the KV-14 SPG, to replace the model 1937 howitzer, and a new single-sided hydro-pneumatic balancing mechanism. 
2. As the power of cannons constantly grows, you may be interested in our installation of an OBM-43 barrel, with the muzzle velocity of 880 m/s and a 43.5 kg shell, on the KV-14 SPG, instead of the 122 mm barrel with the 1000 m/s muzzle velocity. Such a solution raises the power of the SPG by 33.5% and increases its armour piercing and concrete piercing capability.

With this solution, not only does the SPG receive a more powerful gun, but the gun is already in use, and will ease production and use of the guns."

KV-14 is, of course, the pre-production index of the SU-152. 

The document then provides comparisons between the different gun options: the ML-20S, 122 mm OBM-50 (BL-9) and the yet unnamed 152 mm gun. The ranges at 22 degree elevation (max elevation for all guns) are: 12 km for the ML-20S, 21 km for the BL-9, and 18 km for the new 152 mm gun. The penetration for both experimental guns at 1500 meters is 195 mm against flat armour, and 160 mm against armour angled at 30 degrees. This is pretty close to the figures given in that other penetration table (197 mm against flat and 160 mm against angled armour). All three guns have the same horizonal traverse: +/-7 degrees. They can carry 16 shells (38 for the new guns with a redesigned fighting compartment). The projected rate of fire is 1.5 RPM for the ML-20S, 2 RPM for the BL-9, and 1.33 RPM for the 152 mm gun. The mass of the SPG is 47 tons with the ML-20S, and 48 tons with either of the new guns.

However, the OBM series wasn't only destined for tank destroyers. A 122 mm OBM-51 gun, almost equivalent to the OBM-50 (BL-9), was planned for a heavy tank. The project requirements were:

• 1000 m/s muzzle velocity
• 200 mm of penetration at 1000 meters
• Use of existing A-19 shells
• Maximum compatibility with ML-20S components

CAMD RF 38-11369-286: BL-9 against a 203 mm plate at 1000 meters. One shell penetrated completely, 4 shells are stuck in the armour, 3 shells left dents.

You may be thinking of the IS, but no, the tank that was first meant to carry this powerful gun was the KV. On October 26th, 1943, the Technical Department of the NKV held a meeting on the 122 mm OBM-51 tank gun proposed by OKB-172. The details are contained within CAMD RF 38-11355-1403.

"As ordered by the Tech-department of the NKV, OKB-172 developed a project for a modernization of the KV tank by installing a 122 mm gun. The modernization removes the current KV turret and all its armament, and replaces it with a new turret that holds a 122 mm high power gun, two machine guns, and the ammunition rack. The rest of the tank, all of its mechanisms, components, and armoured hull, including the turret ring, remain the same.

The 122 mm gun designed by OKB-172 has the following characteristics:

1. Caliber: 122 mm
2. Muzzle velocity: 950 m/s
3. Shell mass: 25 kg
4. Maximum gas pressure: 2660 kg/cm^2
5. Barrel length with muzzle brake: 58.5 calibers
6. Recoil resistance: 16 tons

The tank's turret consists of homogeneous rolled armour plates. Front: 75 mm, side: 60 mm, roof: 30 mm. In order to balance out the gun, the rear of the turret has 150 mm of armour. 

The gun is installed in the front of the turret on a welded mount. The mount and vertical aiming mechanism are borrowed from the SU-152 SPG with no changes. In order to balance the oscillating part of the gun, a hydromechanical balancing mechanism is planned. The turning mechanism is the same as in the KV turret. The tank carries 45 rounds of ammunition in the turret and on the bottom of the fighting compartment.

After discussion, we have come to the following conclusions:

1. The modernization of the KV tank to equip it with a powerful 122 mm gun is a modern and reasonable course of action.
2. The project is approved to continue to the prototype stage.
3. It is reasonable to also adapt the modernization project, including the turret, to the IS tank.
4. During development, it is suggested to:
1. Use the D-5 mount, as it is the most compact.
2. Use a horizontal sliding breech. 
3. Increase the muzzle velocity to 1000 m/s on the IS chassis.
4. Develop a cast turret variant, and make it as short as possible.
5. Reduce the ammunition capacity to 30 shells to reduce turret height and mass.
6. Allow for shooting on the move, move the fire button to the elevation mechanism. Implement a secondary electric fire mechanism.
7. Reduce the tank mass to 45 tons.
5. Give OKB-172 the highest priority, given the importance of this project.
6. Suggest that OKB-172 work with naval armament factories."

As we already know, the lower velocity D-25T was chosen as the gun for the IS. Its penetration was entirely adequate for the German tanks that ended up being produced, and the high power 122 and 152 mm guns were ultimately not needed.