ISU-122S

Description

The ISU-122S was an improved design (rather than a separate variant) of the ISU-122 assault gun used by the Soviets during World War II, mostly in the anti-tank role. A semi-automatic breech block was fitted to the original 122 mm A-19S tank gun. The tank gun with this new design was designated as the 122 mm D-25S. The modification resulted in an increase in the fire rate to 3 rounds per minute from 1.5 rounds per minute. The ball-shaped gun mantlet and double-baffle muzzle brake of the ISU-122S are distinguishing features. The Germans coded the ISU-122S as Type 249 in their official reports.

Introduced in the Closed Beta Test for Ground Forces before Update 1.41, the ISU-122 is a long-range sniper that performs well in combat due to its high penetration, accuracy, and good reverse speed. Locate a good spot to snipe from and engage the enemy from a distance to keep them from approaching the objectives. Due to the tall profile and inadequate frontal armour, one should always stay away from the frontline to avoid being flanked or swarmed. Although there is a significant reduction in reload time, players should still make their shots count since most enemies reload much more quickly, making it rare to be able to take a follow-up shot if the initial shot misses.

General info

Survivability and armour

The ISU-122S's armour profile is almost identical to that of its predecessor, except for a different gun mantlet. The armour will protect against smaller calibre guns and autocannons, but should not be relied upon against most tanks it faces at its battle rating. The gun mantlet is the strongest part of the armour, and, because the gun is offset to the right, it is thus recommended to try and hide the left side and only expose the right side of the front towards enemies.

The relatively thick side armour can cause bounces and ricochets if angled, but depending on this is risky for a turretless vehicle, as it may get its tracks disabled and rendered helpless.

Armour type:

• Rolled homogeneous armour (hull, superstructure/casemate, roof)
• Cast homogeneous armour (gun mantlet)

Notes:

• Suspension wheels are 20 mm thick while tracks are 30 mm thick.
• There is a 40 mm plate under the gun mantlet, reinforcing the original 65 mm gun mantlet case.

Mobility

Based on the IS-2, the ISU-122S has its robust chassis, wide tracks and powerful engine. It benefits thus from the same good mobility despite being a relatively heavy vehicle. The acceleration is good and it will reach its maximum speed without any problem.

The very high reverse speed allows it to back off from an engagement quickly, once shot at or spotted. The wide tracks give it good handling and performance on soft terrain. The ISU-122S is quite sluggish in stock condition, especially when turning on the spot. And because of its weight, it is slow when going uphill. If you have to turn quickly on the spot, privilege using your reverse gear because it has more torque.

Modifications and economy

• Start with Parts and FPE for the survivability.
• Proceed with Tracks to increase your mobility.
• After that, prioritize either firepower or mobility modules depending on your needs.

Armaments

Main armament

Horizontal traverse arc of the ISU-122S

The 122 mm D-25S comes from the same design as the 122 mm D-25T equipped on the IS-2. They have the same ballistic characteristics and the same available ammunition choice. The S stand for "samokhodnaya" (self-propelled) while the T stand for "tank", the main difference being the length of the breech: short breech for the IS-2 in its cramped turret and long breech for the ISU-122S because of the casemate layout.

Being originally an artillery field gun, the gun uses two-piece ammunition with projectiles and propellant bags. These combined with the large calibre of the gun result in a relatively low rate of fire. This causes the reload time to be 2 to 3 times longer compared to vehicles at the same BR. However, because of the improved breech, the ISU-122S still reloads faster than the preceding ISU-122 and the IS-2s.

The gun remains reliably accurate until 1,300 m distance. Beyond 1,000 m, the poor optics will limit its accuracy as the sight magnification is poor. The high muzzle velocity of the shells result in relatively flat trajectories and thus helps hitting moving targets from a distance. Using the "sight distance control" feature can increase your chances of successfully hitting enemy tanks.

The gun has limited horizontal traverse. The traverse arc is asymmetrical due to the commander's position limiting rotation towards the left side. This can make tracking a moving target difficult. This should be taken into account when positioning the vehicle. Additionally, because of the size of the gun and the manual traverse, the gun traverse speed is very slow compared to other tanks at the same rank or battle rating. The result is that the ISU-122S is rather slow to react against moving targets, and being able to predict a target's movement is a skill that will really help overcome these limitations.

The ISU-122S also has very poor gun depression and will struggle to bring its gun to bear on a target on all but the flattest of terrain. Gun elevation, on the other hand, is quite good. This can be used to make up for the poor gun depression to some extent with creative positioning.

Your recoil is important but not powerful enough to throw your gun off target after firing: the damping cylinders on the gun are too weak to absorb most of the recoil power and the suspension will absorb much of it. Your reload time being very long, the recoil is not as much of a handicap as it could be.

Ammunition

The available choice of ammunition focuses on heavily armoured targets:

• BR-471: APHE; an armour-piercing shell with high explosive mass and high muzzle velocity that will one-shot any tank that it penetrates.
• BR-471B: APHEBC; essentially the same as the BR-471, but with significantly improved performance against sloped armour thanks to the ballistic cap. This shell can reliably penetrate the frontal glacis of a Panther from over 700 m. The preferred shell against armoured targets.
• OF-471: HE; most effective against lightly armoured and open-topped vehicles. However, the projectile contains enough explosive to penetrate the roofs of some tanks via the blast overpressure.

Ammo racks

Ammo racks of the ISU-122S

Notes:

• The ISU-122S uses two-piece ammunition, composed of propellant bags (orange) and projectiles (yellow). Both have separate racks.
• To go into battle with the right flank depleted of propellant bags (racks 1 to 3), pack 17 (+13) shells.

Machine guns

The 12.7 mm DShK heavy machine gun is located on a pintle mount, manned by the loader, and can act as both an offensive and anti-aircraft machine gun on the ISU-122S, giving protection against aircraft and lightly armoured vehicles. Effective range is within 500 m against lightly armoured vehicles. The 360° traverse allows you to point the DShK in any given direction. However, the position of the DShK high on the casemate and the poor depression angle make it difficult to shoot at low-profile targets at close range. The DShK also has a slow traverse speed compared to the American .50 cal M2HB mounted on American-made vehicles.

Usage in battles

Offensive role

The ISU-122S performs well as a long range sniper, taking advantage of the APHEBC shell's high penetration values, high gun accuracy and good reverse speed. Find a shooting spot and engage targets from at least 1,000 m away to prevent enemy progress towards the objectives. Because of its high profile (casemate), long reload, and weak frontal armour, always stay away from the frontline to avoid being flanked or swarmed. While your gun is very powerful, your in-game performance will highly be dependent on the map you are playing on. Open maps like Fire Arc, Kursk, or Maginot Line will allow you to make the best use of its sniping capabilities, whereas urban environments like Advance to the Rhine are generally poor maps for the ISU-122S. In case you are playing on a mixed terrain map, prefer open spaces.

Defensive role

In case of an enemy breakthrough, your role is to contain the enemy advance by pinning them down. The enemy will be moving the front line, so engagements will take place at a closer range than you're comfortable with. You should identify and target priority choke points, narrow passages, and corridors of the map that the enemy will be forced to take. Your limited gun traverse only allows you to monitor an area spanning about 175 m at 1,000 m distance without having to reposition your hull. Position the left side of your casemate behind an obstacle to leave only your gun mantlet exposed, the thickest part of the frontal armour.

Other roles

If you try to use the ISU-122S in a different role on the battlefield, you will be confronted with the following:

• If you try to play as a front-line tank, you will be quickly disabled by any type of fire coming from enemy tanks. Your frontal armour is pretty thin for a tank to play at close range, and a successful penetrating shot from the front will usually disable your crew sitting in a line. Your gun traverse is very limited and your turning speed is also too poor to manoeuvre quickly enough in close combat.
• If you attempt to play support tank, your high profile will make it difficult to stay hidden behind obstacles without taking fire. Your good reverse speed can be an advantage but the enemy will flush you out with an artillery strike (which can be accurate enough at medium range to disable you). Even with an escape route planned and cover to hide behind, your chances of survival are meagre. You will also be at risk of being flanked by light tanks or armoured cars, especially if they swarm you. Anyone who manages to get behind you has a very high chance of destroying you. In that role, an undetected flanking enemy, artillery strikes or CAS will be great threats to the ISU-122S.

Notable enemies

• Any tank will disable you with ease in a close range encounter because of your weak unsloped casemate armour.
• For long range engagements, commonly met guns able to penetrate the ISU-122S from 1,000 m are:
  • USA: tanks equipped with a long 76 mm gun (M4/M41), M36 GMC
  • Germany: any vehicle equipped with a 75 or 88 mm gun like Panthers, Tigers I & II, etc.
  • USSR: any vehicle equipped with an 85 or 122 mm gun like the IS-2, T-34-85, etc.
  • Great Britain: any vehicle equipped with APDS like the Centurion, Challenger, and Charioteer.
  • Japan: Ho-Ri and US-built tanks.
  • China: vehicles similar to the US and USSR ones.
  • France: ARL-44 and AMX-13.
  • Sweden: Ikv 103 and Strv 74.

Differences between non-volumetric shells and volumetric shells hitting a Tiger I

An important note when engaging tanks, especially Tiger I's, is that, with the implementation of volumetric shells, you can no longer expect your shells to pierce through small openings or right past an obstacle. The large 122 mm calibre of your shells will result in it catching on an armour plate's edge very often, losing all of its penetration. For example, a Tiger I has some edges that can absorb your shell unexpectedly, despite having flat armour. Thus you must always make sure that where you are aiming at is absolutely free of obstruction, or it may result in a non-penetration or a ricochet. This can be seen in the diagram to the right.

Defeating an ISU-122S

• Wait for ISU-122S to shoot before engaging it: its long reloading time is a big weakness as it is defenseless for at least 15 seconds like an IS-2. It can however retreat quickly thanks to its good reverse speed. Be careful not to get baited into open field trying to follow your prey.
• The ISU-122S has a rather thin armour when compared to tanks at the same BR: if you have a very high penetrating power, aim for the casemate.
• In a frontal encounter, the right side of the frontal armour plate is the biggest weak spot. A successful penetration will most likely knock out 3 out of 4 crew members because they sit in line along that axis.
• In a flanking manoeuvre, the center of the casemate is the place to shoot to knock most crew members out.
• From behind, the left side of the casemate is the best spot to disable the ISU-122S.
• If you are alone, flanking a ISU-122S undetected remains your best chance to destroy it. But it is difficult because the ISU-122S ideally positions itself very far from the frontline.
• Force it out of position with indirect fire (artillery strikes) if you're at relative close range.
• In case of a frontal attack (choke point/corridor), try blinding the ISU-122S with smoke and then swarm it with several tanks to saturate its defensive capacities.
• Engage the ISU-122S while it is moving: it takes a long time to brake, turn, bring the gun to a stable position and target precisely. With a reload time over 15 seconds, no player will take a shot that has little chance to hit.
• Do not take pot shots or shoot on the move, aim precisely at the weak spots (casemate walls). Your chance of ricochet is increased at long range as your penetration power decreases and the ISU-122S can angle its hull in a defensive manoeuvre.

Pros and cons

Pros:

• Extremely potent 122 mm gun.
• Reloads significantly faster than other 122 mm gun platforms.
• Large gun mantlet can be hard to penetrate.
• Decent all-round protection against autocannons.
• Powerful roof-mounted MG that can rotate at 360°
• High reverse speed.

Cons:

• Long reload time compared to most contemporary vehicles.
• Unsloped frontal armour that is insufficient against commonly encountered tank guns.
• Cramped crew compartment results in poor survivability.
• Very poor gun depression and narrow traverse arc.
• Prominent gun breech often gets damaged.
• Vulnerable when immobilized and flanked

History

Development

When the ISU-152 began production in 1943 with the large 152 mm ML-20S gun, production of the chassis and hull soon exceeded the supply of the ML-20S gun, which caused a decrease in the production of self-propelled guns for the Soviet military. It was determined to speed up self-propelled gun production, the ISU chassis were to be mounted with the 122 mm A-19S gun. Work on this mounting already began before the supply issue in December 1943 at the Chelyabinsk Kirov Plant (ChKZ), which had a design team simply take the ISU-152 chassis and hull and only changing out the armament with the 122 mm gun, but this prototype - Object 242 - was not immediately put into production despite successful testings. When the supply issue arose, the prototype was then adopted by the Soviet Defense Committee for increased production and also that the 122 mm gun had better accuracy against tanks than the 152 mm gun. The ISU armed with the 122 mm gun, designated the ISU-122, began production in April 1944.

Upgrade

The original ISU-122 is equipped with the 122 mm A-19S gun, which had a firing rate of 1.5 to 2.5 shots per minute due to its manual-piston breech. An attempt to fix this was a modernized A-19 gun called the D-25. The D-25 didn't see instalment onto the ISU-122 for a while due to prioritization on the IS-2 tanks, but it was available for use on self-propelled mounts after September 1944 as the D-25S. The ISU-122 prototype mounting the D-25 gun was called Object 249 which was successfully tested and an improvement over the A-19 gun with an increased firing rate of 2 to 3 shots per minute with one loader and up to 4 shots per minute with an additional loader in the crew. The D-25 also exploited the use of a muzzle brake in order to reduce the recoil forces on the gun, so the recoil buffer on the D-25 was much smaller than the A-19, which helped reduce the size of the gun and improved crew workspace in the self-propelled gun. Object 249 was then adopted as the ISU-122S, which entered production side by side with the ISU-122 u and both were produced up until the end of 1945. ChKZ produced both vehicles during the war and manufactured 1,735 ISU-122s (April 1944 - December 1945) and 675 ISU-122Ss (September 1944 - December 1945).

Combat usage

The ISU-122 were organized in units in a similar fashion as the ISU-152. The focus was made to not mix the two vehicles in the same regiments or brigades due to logistical reasons with ammunition and ballistic differences in indirect fire, but some units were equipped with both vehicles anyways. Like the ISU-152, the ISU-122 proved to be a very powerful multi-purpose vehicle as an assault gun, self-propelled artillery, and tank destroyer. The 122 mm gun on the ISU-122 allowed it to engage armour and fortifications with heavy armour-piercing round and a large high-explosive round. The primary role of the ISU-122 was as a tank destroyer due to the gun's superior ballistics compared to the ISU-152's, which large calibre size more suited its role as an assault gun. The ISU-122 did participate as an assault gun role in urban combat to support the infantry, but the lack of rotating turret and long barrel length restricted its manoeuvring and ability to engage all enemies in a large angle. Nevertheless, commanders found the ISU-122 as a very good assault gun. Usage of the ISU-122 in an indirect firing role as a howitzer is rare, but when used could fire up to 14 kilometres away. Usage as an artillery piece is more likely during a rapid offensive where towed artillery behind the lines is unable to keep up with the advancing infantry lines.

Media

Skins

• Skins and Camouflages for the ISU-122S on live.warthunder.com.

Videos

See also

Vehicles equipped with the same chassis

• ISU-122 / ISU-122 (China)
• IS-2 (Family)

Similar Soviet vehicles

• SU-152
• ISU-152 / ISU-152 (China)
• SU-122-54

Other vehicles of similar configuration and role

• Panzer IV/70(A)
• Panzer IV/70(V)
• Jagdpanther
• Ho-Ri Prototype
• CA Lorraine
• Ikv 103

External links

• [Wikipedia] ISU-122
• Tanks Encyclopedia - ISU-122 & 122S