In 1941, during a phone call to Stalin, Grabin expressed the need for "a powerful tank gun, capable of penetrating armour of the tank that carries it at 1000 meters". Stalin suggested that the gun should be even more powerful than the modernized 107 mm gun. The 107 mm tank gun ZiS-6 was produced in only 38 days. Another 107 mm gun was considered, the F-42, but it was too bulky to install in even the KV-2's roomy turret. However, several parts of its aiming and loading mechanism were borrowed for the ZiS-6.
But that's not the most powerful gun of that caliber that was built. In 1938, a gun of unprecedented power was ordered. 107 mm in caliber, 70 calibers in length, capable of penetrating a 160 mm armoured plate sloped at 30 degrees, from a kilometer away! All this, at 10 rounds per minute.
Let's see how all of these guns fared.