The gun was developed as a variant of the
5 cm Pak 38 towed anti-tank gun using the same ammunition. On the
Panzer III, it replaced the
5 cm KwK 38, which had an L/42 calibre length, less powerful ammunition and a lower muzzle velocity. However, even the 5 cm KwK 39 gun with a longer barrel, higher muzzle velocity and more penetration was not sufficient against newer Soviet
T-34 and
KV-1 tanks. Therefore, as time went on, the Panzer III was no longer effective as a medium tank that could engage in fights with enemy tanks. So, a new role for the Panzer III tank was found. On the Panzer III, the 5 cm KwK 39 was phased out in favor of the shorter but larger calibre
7.5 cm KwK 37 L/24 low velocity guns that could fire more effective
HE and
HEAT rounds. HE howitzer type rounds with 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, though limited, capability against enemy armour. Thus, they were used against enemy tanks mostly in an emergency. With these changes, the Panzer 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. ==Ammunition==