The gun in this machine (intelligent design indeed, considering what was available and the time constraints) is the famous ZIS-3, a 76.2 mm gun that even with it's carriage weighed only 1200 pounds! Amazing.
The armor piecing projectile it fired in it's day as the primary anti-tank gun, called the BR-350A, would penetrate two inches of vertical armor at over a mile. PanzerKampfWagen III and IV would not have a chance.
http://www.battlefield.ru/guns/field_7.html
The Russian AT guns were so good the Germans used captured ones on their SPG's