The Iosif Stalin tank (or IS tank, named after the Soviet leader Joseph Stalin) was a heavy tank developed by the Soviet Union during World War II. The tanks in the series are also sometimes called JS or ИС tanks. The heavy tank was designed with thick armour to counter the German 88 mm guns, and carried a main gun that was capable of defeating the German Tiger and Panther tanks. It was mainly a breakthrough tank, firing a heavy high-explosive shell that was useful against entrenchments and bunkers. In late 1944 the design was upgraded to the IS-3. This tank had improved armour layout, and a hemispherical cast turret (resembling an overturned soup bowl) which became the hallmark of post-war Soviet tanks. While this low, hemispherical turret may have made the IS-3 better protected, it also imposed severe penalties inside the tank by significantly diminishing the working headroom, especially for the loader (Soviet tanks in general are characterized by uncomfortably small interior space compared to Western tanks).