The production ISU-122S heavy assault gun.

The main problem during mass production of the new assault gun was lack of ML-20S cannons. Soviet artillery plants weren't able to produce them in proper quantity. Although there were plenty of 122 mm A-19 cannons and huge number of 122 mm ammunition. As a result, in summer 1943 the SKB-2 (design bureau in Chelyabinsk) has been ordered to check the chance to mount A-19 cannon instead ML-20S.

It was easy because both cannons had the same gun-carriage. At the end 1943, the very first prototype - "Object 242" - has been finished. Then it was tested and accepted for service under the ISU-122 designation. By the end of 1943, the first party of the new ISU-152 and ISU-122 were ready, simultaneously with the first IS-1 heavy tanks.

During the war there were several attempts to increase the firepower of the ISU-122. The A-19 cannon was modernised for usage with IS-2 tanks. For this purpose cannon has been equipped with a new semi-automatic block-breech and renamed as D-25S. After modernisation, the practice rate of fire (don't mix with maximum ROF!) increased from 1.5 to 3 shots per minute.

The prototype of the ISU-152 assault gun.

Soon, the idea to mount D-25S on the ISU-122 was suggested. The new prototype with the D-25S was named "Object 249". This vehicle had a new gun mantlet, also crew's layout was improved. "Object 249" was accepted for service and renamed as ISU-122S (or ISU-122-2) designation.

The ISU-152, ISU-122 and ISU-122S heavy assault guns were based on IS-2's chassis. Most of their mechanisms and devices were unifiewd with the IS-2's ones. Although those assault guns were equipped with different fuel tanks.

During war there were some ideas to rearm these assault guns with various artillery systems: the 152 mm long barreled gun BR-8, the 152 mm gun BL-10, and the 130 mm S-26. Prototypes of all these vehicles have been built, but due to different reasons non of them were accepted for service. Two of them are now displayed at Kubinka. There are ISU-152 with the BL-10 and the ISU-130 with the S-26.

The ISU-122 of the Polish 25th Self-Propelled Artillery Regiment crossing the Neisse river. April 1945. The ISU-122 (in front) and the IS-2 (behind) running through a small Rumanian town. September 1944.

From the spring 1944, the Heavy Self-Propelled Artillery Regiments (the Russian abbreviation is TSAP) of SU-152 being re-equipped with new ISU-152 and ISU-122 assault guns. All those regiments received the "Guards" designation. By the end of the war, totally 56 of such units were formed.

Each regiment had 21 ISU-152 or ISU-122 or a kind of mixture of both types. Both assault guns used for the same tasks. Russians didn't distinguished them and used them for same tactic tasks. The Russians strove do not mix them in same units though.

In March 1945 the 66th Guard Heavy SP-Artillery Brigade has been formed. It was organized on three regiments: totally 1804 men, 65 ISU-122 and 3 SU-76M.

In February 1944, the very first units with ISU's were formed. They were named as the tank units, for example: Independent Heavy Self-Propelled Artillery Regiment (OTSAP - "Otdelniy Tyazheliy Samokhodno-Artilleriskiy Polk"). OTSAP consisted of four batteries (5 vehicles per battery plus one vehicle for regiment's commander). The tactic employment was the same as for common Heavy Tank Regiments: support the offensive operations. All OTSAPs were in Army's and Front's subordinate (mostly in reserves). Those assault guns used mainly for the long range gunfire because they weren't effective at short ranges. The slow ROF did them ineffective in close combat.

The ISU-122 become the most effective long-range tank destroyer, while the ISU-152 had the average armor penetration. That was because of absent of AP ammunition and slower muzzle velocity. That's why many western sources consider the ISU-122 as a "tank destroyer" and ISU-152 as an "assault gun". In fact, both guns were heavy assault guns, cause they used in this manner (as I said above, Russians used them for the same purposes). ML-20S was able to penetrate the 120 mm vertical armor from the 1000 metres, A-19 - 150 mm armor from the same distance. In spite of absent of machine-gun(s), all three types of these assault guns were very effective weapon for a street fighting because of powerful guns with a great HE effect.

The ISU-152 on the street of Polotzk (Byelorussia). July 1944.

The first large-scale usage of the new assault guns was in summer 1944, during the Soviet offensive operation "Bagration", where at least 14 OTSAPs were involved. Soon, these guns acquired the honor of very effective weapon against German "Tigers" and "Panthers". For example, in summer 1944, the half of the 502th PzAbt (6 vehicles of 12 total) was destroyed by ISU's gunfire.

Often those Heavy Self-Propelled Artillery Regiments used together with tanks or infantry and supported their attacks. Usually those ISU's become the main defensive weapon during sudden German counter-attacks.

For example, on January 15, 1945, in East Prussia near the Borove town, Germans launched a counter-attack against Soviet infantry that was supported by 390th Guards OTSAP. Soviet infantry retreated behind the regiment's positions. The regiment opened a direct fire, many of the German AFVs were destroyed and most of German soldiers were killed. The counter-attack failed.

The ISU-122S assault gun. Konigsberg. April 1945.

Time to time, ISU's used for artillery missions because they were able for both direct and indirect fire. In particular, during the Sandomir-Silesia offensive operation, the 368th Guards OTSAP of the 1st Ukrainian Front was assigned for artillery mission and for 107 minutes the regiment fired 980 shells.

As a result, two German mortar batteries were destroyed as well as other 8 German guns, about one German infantry battalion was wiped out. Interesting to note, that additional rounds were placed at the vehicles, but anyway, ISU's used their "internal" ammunition first. Otherwise, rate of fire become slower because it takes up to 40 minutes to load 20 rounds into the vehicle.