The ISU-122 (acronym of Istrebitelnaja - or Iosif Stalin-based - Samokhodnaya Ustanovka 122) was a Soviet assault gun used during World War II, mostly in the anti-tank role.

After completing development of the ISU-152, ChKZ engineers mounted the A-19 gun on the ISU-152 chassis to create "Object 242" — the first ISU-122 prototype.

A further advantage of rearming the ISU was increased direct fire range against heavy German tanks.

For these reasons, the State Defense Committee adopted Object 242 for Red Army service as the ISU-122 on 12 March 1944.

Soviet designers developed the D-25 by modernizing the A-19S's breech, creating a semi-automatic variant of the 122-mm gun.

The prototype ISU vehicle, armed with a D-25S was designated "Object 249" and successfully passed plant and state testing.

This improved the crew's work conditions and allowed for a smaller, lighter gun shield with the same armour thickness.

However, the original ISU-122 remained in production (along with the ISU-152) due to a large stock of A-19 guns (the ML-20 and D-25 came directly from artillery factories).

[2] After World War II, many surviving ISU-122s were rebuilt as very large calibre gun chassis or supply vehicles.

[citation needed] Some vehicles were fitted with a 12.7 mm DShK anti-aircraft machine gun.

Soviet Army commanders tried not to mix the ISU-122 and the ISU-152 in one regiment or brigade, although there were some units equipped with both types.

An improved version, the BR-471B (БР-471Б), was developed in early 1945, but was available in quantity only after World War II ended.