The Stug III started out as self-propelled infantry support tank but became on the most successful Tank Destroyers of World War 2. Its German designation was Sd.Kfz. 142/1.
The first versions of the Sturmgeschütz tanks had, like the early versions of the Panzerkampfwagen IV, the short 75 mm gun. Which was were underpowered in tank battles. In the western battles tank battles were not that common in 1940 but in the Soviet Union the Panzer amies encountered the T-34 and the KV-1 with sloped armor. The Short barrel L/24 with a length of 176 cm had a penetrating capacity with the Panzergranate 39 at 500 meters of 39 mm. The later StuG versions with the 7,5 cm StuK 40 L/48 with a barrel length of 385,5 cm had at the same distance of 500 meters and the same Panzergranate 39 an amor penetrating capacity of 91 mm. The Panzergranate is an German amor piercing shell.
At 1000 meters the Panzergranate 39 did not exceed a height of 2,50 meters, so a T34 with a height of 2.76 meters within a range of 1000 meters could be fired upon with a flat trajectory shot and a hit was far more easy to accomplish. On the G version side skirts were added as protections from anti-tank riffles and hollow charges.