As the assault guns were more economical to manufacture than the tank versions, and their firepower was equal or superior, more heavy assault guns were built on the IS chassis than the basic heavy tank variants.
The first heavy assault gun on the IS chassis was the ISU-152 which was intended to replace the SU-152.
It was armed with the same ML-20S gun. but the superstructure was higher and more spacious to permit a more efficient operation of the gun by the crew.
Due to shortages of 152mm howitzer tubes, and a surplus of both 122mm A-19 model 1931/37 gun tubes and ammunition, the Red Army decided to arm a portion of the heavy assault guns with the 122mm gun.
The initial production batches from January 1944 to the end of the summer of 1944 were armed with the A-19S gun in a mounting identical to that on the ISU-152, with vehicles still in service being upgraded with the 12.7mm DShK AA hMG from October 1944 on.
There were three regiments with 5th Army and two with 49th Army, both part of an envelopment of the Byelorussian capital of Minsk.
Eight regiments were honored by having liberated cities added to their unit name, three received the Order of the Red Banner and three the Order of the Red Star.
The ISU-122 and ISU-152 also earned the reputation of being the deadliest enemy of the German Tiger I tanks.
For example, of the twelve Tiger I tanks of sPzAbt 502 destroyed in the summer 1944 fighting in Byelorussia and the Baltic coast, about half could be attributed to ISU-122s or
ISU-152s.
To the end of the war CHKZ were released 2159 ISU-122, and by the end of the war, the Red Army had about 2,700 ISU-122 and ISU-152 heavy assault guns in service.
Heavy assault gun production had totaled about 4.800 vehicles of which 2300 or 46% had been lost in combat.
Sources : S.Zaloga, "Stalin's Heavy Tanks", Armor at War #7012, Concord Publ., and others.
This gun, a modernized A-19S, was essentially the same as the D-25T gun used in the IS-2 tank, so it was fitted with a muzzle brake.
In addition, it was placed in a new rounded mantlet of 120-150 mm cast steel which had superior traverse.
Due to the more compact breech of the gun, it was possible to increase the unsufficient rate of fire.
The maximum distance of direct fire was 5000 m, the same as with the A-19S gun.
To the end of the war, CHKZ released almost 700 ISU-122S.
Sources : Baryatinskiy, »Heavy SPG of the Red Army«, Armour Collection #2/2006, and others.
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Last Updated: July 04, 2008