It was developed as a variant of the 5 cm PaK 38 towed anti-tank gun. On the Mark III tank it replaced the 5 cm KwK 38, which was L/42 length in caliber and had a lower muzzle velocity. However, even the 5 cm KwK 39 gun with a longer barrel, higher velocity and more penetration was not sufficient against newer Soviet T-34 and KV-1 tanks. Therefore, as time went on, the Mark III tank was no longer effective as a medium tank that could engage in fights with enemy tanks. So a new role for the Mark III tank was found. On the Mark III, the 5 cm KwK 39 was phased out in favor of the shorter but larger caliber 7.5 cm KwK 37 L/24 low velocity guns that could fire more effective HE and HEAT rounds. HE howitzer type rounds with high explosive forces and shrapnel were very effective against infantry, machine gun nests and towed enemy guns on the battlefield. However early HEAT rounds were somewhat unreliable. They were useful against hardened fortifications and had a good but limited capability against enemy armor. Thus they were used against enemy tanks mostly in an emergency! With these changes the Mark III with the 7.5 cm KwK became an infantry support tank late in its career, while the new, much more capable 7.5 cm KwK 40 L/43 was mounted on the larger Panzer IV Ausf. F2 (and the longer L/48 on subsequent Ausf. versions) to fight the KV and T-34 tanks.