There has been some information lost or ignored (a least apparently outside Germany) concerning the 75mm German guns especially the 7,5 cm Kannon 40 . I'll try to set some of the record straight with compiled information on this page. In different forms the Kannon 40 became to be known as either KwK, StuK or PaK 40.
1. The German Army, shocked by their encounter with Soviet T-34s and KV tanks in Russia in 1941 finally gave priority to the development of more powerful guns. Contract to develop a new 75mm cannon was underway by Krupp. The Kannon model 40 originally was named Kannon 44 and was 40 calibers in length. In short order it was renamed Kannon 40 and its length increased to 43 calibers. The cartridge design was referred to as a 'bottle cartridge' shortened for ease of handling inside a tank. A standard cartridge style was used for an anti-tank gun based on the similar gun produced by Rheinmettal-Borsig for the Kannon 40/L46.
2. Early tests with the Kannon 40 found that a spent cartridge would on occasion fail to eject. This apparently was the higher charged L48 round with a MV of 790 m/s. (Though it is unclear which gun this happened to when mentioned in Spielberger's book on the Sturmgeschutz.) The solution to the ejection problem was to reduce the charge and thus the muzzle velocity of the round. Somewhere along the line the muzzle velocity of the L48 was reduced from the initial 790m/s to 750 m/s. It seems that it took about a year before the charge was reduced when the ejection jams were noted during development testing.
3. The 75mm/L46 Pak 40 having a different style cartridge kept its muzzle velocity at the design 790 m/s for a longer period of time. Though later in the war, for some reason, maybe training or consistency, the charge was reduced and the muzzle velocity became 750 m/s as well. Download site
4. In late 1943 or early 1944 tests conducted by the Russians with captured model 40 75mm guns lists the AP muzzle velocity as 770 m/s. This coincidentally is an average of 750 and 790 m/s. They could of just averaged the readings of a mixture of old and new shells. In 1944 a US document TM-E 30-451 on German 75mm guns also gives the muzzle velocity of the PaK 40 as 770 m/s. The wiki says at one point an official German document does have the MV at 770 m/s. So there may have been an intermediate power cartridge for a period of time. In fact Panzer Worlds Christian Ankerstjerne found that the 7,5 cm PaK 40 had two different powder loads.
D 435/1 (December 1942): 2750 kg Digl R P - G 1 - (625*3,8/1,3)
H Dv 119/324 (October 1943/October 1944): 2690 kg Digl R P - G 1-(625*3,8/1,3)