Though Russian JS-2 heavy tank was equipped

with 122 mm A-19 gun, it was used mainly as "bucker buster" because large piece of HE (high-explosive shell) fire.  Its AP (armor-piercing) shells, however, were ineffective to the armors of German heavy tanks such as Panther tanks. 

Besides, the two-piece ammunition was difficult to handle and slow to reload (the rate of fire was only about two rounds per minute). Another limitation imposed by the size of its ammunition was the payload: only 28 rounds could be carried inside the tank.^[4]