Creighton

“constituted as much as one third of the divisions’ combat strength, were seriously under-gunned (with machine-guns and 20mm main guns) and, therefore, unable to contend with new model Red Army tanks such as the T-34 and KV. On the other hand, although the dependable second-generation Pz. III and Pz. IV tanks were more than a match for the older Soviet tanks, such as the T-26 light, T-8 medium, and T-35 heavy models, even they experienced difficulty destroying T-34 and KV-1 and 2 tanks. In 1941 Germany was in the process of re-arming all Pz. III’s with a medium-velocity 50mm main gun, while the Pz IV’s still retained a low-velocity 75mm gun. The velocity of these weapons was at least as important as the size of the shell because high velocity was necessary for effective armor penetration. Neither German weapon could penetrate the thick frontal armor of the T-34 medium tanks and KV-1 heavy tanks that were just coming off the assembly lines in Russia.”
― David M. Glantz, Barbarossa Derailed: The Battle for Smolensk 10 July-10 September 1941: The German Advance, The Encirclement Battle, and the First and Second Soviet Counteroffensives, 10 July-24 August 1941