Object 701-1, IS-4 Prototype with a.....small mortar strapped to the roof?

Would you like to see the Object 701-1 ingame?
  • Yes.
  • No.

0 voters

How would you like to see it added ingame?
  • As a researchable vehicle.
  • As a premium vehicle.
  • As an event vehicle.
  • As a squadron vehicle.
  • I said ‘No’ in the first question.

0 voters

Hello everyone! Today I’m going to suggest the second Prototype of IS-4!. This is the Object 701-1!

The Object 701-1 while being tested. Note the small 50mm mortar sticking out the turret roof

Quick note:

The name Object 701-1 is more an English version of the original name of the tank. The actual name is Объект 701 № 1.
As you can see in Russian the Prototypes are given a number, and this one is Number 1. But on the English side of the internet the 701-1 seems to be more widely used. And me having to constandly say Number 1, or No 1 might get a bit confusing. So I’m using Object 701-1 to be as clear as possible.

And incase you are wondering, there are seven total IS-4 Prototypes! From the Object 701-0 up to the Object 701-6!

With that out of the way, let’s continue!



The story of this tank began in 1944, she was the second IS-4 Prototype made. If you want to read more about the origins of the IS-4 Prototypes then check out my post on the first Prototype:

This tank (The Object 701-1) was made by the Kirov plant in the summer of 1944. This tank served as a test platform for futher testing of the Object 701, and the Object 701-1 was fitted with something special that none of the other IS-4 Prototypes had.
That being a 50mm S-11 mortar fitted in the roof of the turret, just above the loader. The idea was that the mortar could be used for self defence against enemy infantry. But just like many weird ideas designers put on prototype tanks, this mortar did not seem to impress many people.

Design wise the tank seems to have been the same as the previous prototype (The Object 701-0). So they are very similar to eachother.

The Object 701-1 being tested

The tank went out for testing on the 27th of June 1944. She managed to drive 1103km over the course of her being tested. The average speed achieved by the tank during this time was 20,9 km/h. This was not the maximum possible speed, but just the speed the tank could get up to in normal situations.

During all these driving trials the tank suffered from mechanical malfunctions. One of the bigger issues being the engine. The 750hp V12 was not exactly reliable during these early testings. There were problems in the cast-iron engine blocks, fuel lines, failure in the fuel pump and to top things of, excessive overheating of the engine.

Many of these issues were to be fixed later on as the Object 701 program continued, but it would still take some time.

The Object 701-1 had the same main armament as the previous prototype, that being the experimental 122mm S-34-II gun.

The 122mm S-34-II was an experimental gun based on the 100mm S-34 gun. It was made as replacement for the D-25T. The S-34-II had a few big improvements compared to the D-25T:

  • The gun was more rigid and allowed for more accurate fire.
  • The lack of a muzzle brake reduced the size of the smoke comming out of the gun when firing, and also made it safer for infantry to be standing next to the gun when firing.
  • The gun was fitted with a projectile rammer which increased the rate of fire.
  • A device was fitted that would blow air through the gun after it had fired, which reduced gas contamination inside of a tank turret.

So with all those improvements, why was the gun not used on any other tanks? Well that is because this gun required the gunner to be on the right of the gun, and the loader on the left. And on all other tanks with the D-25T it was the other way around. So this would have required a redesign for any tank that wanted to use this gun. And in the end it was cheaper to use the D-25T instead of redesigning all tanks with 122mm guns fitted.

In the end the tank was deemed quite succesfull. It proved to be superior armour wise to any other tank currently in service with the Soviet Army. Futhermore the maneuverability and armament was also seen as better then expected. And so the Object 701 program continued on with futher prototypes.

The final fate for the Object 701-1 was to be taken apart and be re-build into the Object 701-5, which was one of the last prototypes made and had many improvements to both armour and reliability.

A front view of the Object 701-1



I would love to see this, or any other, IS-4 Prototype ingame. They look really cool with that different turret and these Prototypes are the only tanks ever fitted with the S-34-II gun, so it’s the only way we can see that gun ingame!

This tank (The Object 701-1) is quite unique compared to the other IS-4 Prototypes, that being the 50mm mortar. Not exactly an usefull tool ingame, but still unique nonetheless.

I think this tank would make for a cool squadron vehicle, but let me know what you think in the poll above!

To wrap things up let’s compare this one to the ingame IS-4M:

Object 701-1 <|> IS-4Minnmm

4** <|> 4**
750hp V12 <|> 750hp V12
Suspension Type
Torsion bar <|> Torsion bar

Main armament
122mm S-34-II <|> 122mm D-25T
Vertical guidance
-3° to +19° <|> -3° to +19°
Rate of fire (Rounds per minute)
2,3 - 4 <|> 2,2 - 3

Secondairy armament
50mm S-11 <|> Nonemmm

Auxiliary armament
mmmi7.62mm DT <|> 2 x 12.7mm DShK

Turret armour
~400mm <|> ~400mm
170mm <|> 250mm
170mm at 20° <|> 200mm at 35°
160mm at 30° <|> 170mm at 35°
30 - 30mm <|> 30 - 50mm

Hull armour
Frontal Upper Glacis
120mm at 61° <|> 140mm at 61°
Frontal Lower Glacis
160mm at 20° <|> 160mm at 45°
Upper Sides
160mm at 21° <|> 160mm at 30°
Lower Sides
160mm at 0° <|> 160mm at 0°
Rear plate above the Transmission
120mm at 38° <|> 100mm at 37°
Rear Upper Glacis
120mm at 32° <|> 100mm at 32°
Rear Lower Glacis
120mm at 30°*** <|> 100mm at 40°***
Engine Deck
30mm <|> 30mm
30mm <|> 30mm

** = On the IS-4M the gunner and commander sit to the left of the gun and the loader to the right. This is swapped on the Object 701-1.
*** = These two armour layouts both amount to almost the same effective thickness, but a plate at a higher angle has a higher chance to ricochet an incomming shot.



Armour layout of the Object 701-0, which the Object 701-1 is identical to

Basic stats:

  • Crew: 4 (Commander, Driver, Gunner, Loader)
  • Powerplant: 750hp V12
  • Suspension Type: Torsion bar
  • Turret traverse while targeting: 7,5 degrees/second
  • Maximum possible turret traverse: 15 degrees/second


  • Main armament: 122mm S-34-II gun
    • Vertical guidance: -3° to +19°
    • Rate of fire: Around 3-4 rounds per minute
    • Ammunition storage: 30 rounds
    • Ammunition types: Most likely the same as the D-25T, just not the post war BR-471D round.
  • Secondairy armament: 50mm S-11 mortar
    • Ammunition storage: 50 rounds
  • Auxiliary armament: 7.62mm DT machine gun


  • Turret:
    • Mantlet: 400mm effective armour when hitting it above the barrel
    • Front: 170mm at various angles
    • Sides: 170mm at 20°
    • Rear: 160mm at 30°
    • Roof: 30mm

  • Hull:
    • Frontal Upper Glacis: 120mm at 61°
    • Frontal Lower Glacis: 160mm at 20°
    • Upper Sides: 160mm at 21°
    • Lower Sides: 160mm at 0°
    • Rear plate above the Transmission: 120mm at 38°
    • Rear Upper Glacis: 120mm at 32°
    • Rear Lower Glacis: 120mm at 30°
    • Engine Deck: 30mm
    • Bottom: 30mm

A front view of the Object 701-1, the 50mm mortar is clearly visible

That is all for now! Hope you enjoyed reading about this unique prototype.

Make sure to leave a vote in the poll above!

Also be sure to check out all the other IS-4 Prototypes I’ve suggested:

  • Object 701-0, this is the first IS-4 prototype.
  • Object 701-1
  • Object 701-2, this one is armed with a 100mm gun!
  • Object 701-3, this was used for firing trials. (This thread is not a suggestion, but just a normal post)
  • Object 701-4, this was one of the last IS-4 prototypes
  • Object 701-5, this is the last IS-4 prototypes with the S-34-II gun.
  • Object 701-6, this is the pre-production IS-4.



Primary source:

Secondairy sources:

Extra sources:

S-34-II gun: