German 75mm Model 40 Guns

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)

5.  The first 75mm cannon mounted on German tanks and assault guns was the short 75mm/L24 cannon.   The armor piercing (AP) shell was the Panzer Kannon Grenade with red band (K.Gr rot Pz.).  The shell had a large (by German standards) cavity for an explosive charge of 80 grams. In North Africa a quantity of these shells were captured by the British and subjected to testing.  The Cairo tests during March 1942 showed that penetration of tank armor by the German 75mm AP shell caused a disastrous burst effect inside the tank.  It was decided that thousands of captured German 75mm AP shells were to be converted to be fired by US 75mm guns.  As this was March 1942 and the first Kannon 40 were just being built in March 1942 the German 75mm shells captured could only have been the short 75mm (K.Gr rot Pz.). 

6.  When the first 75mm/L43 guns came out a new type of shell was introduced.  This was the PzGr. 39 red.  Or with red band.  This shell may have had a large charge like the earlier 75mm (K.Gr rot Pz.).   

7.  The kannon 40 7.5cm/L43 fitted to the Panzer IV F in 1942 became the Panzer IV F2.  Later a more powerful gun, the 75mm/L48 replaced the L43 midway through the production of the Panzer IV G.

8.  After the battle of Kursk the Russians ran some tests on their T-34s to see what was causing catastrophic explosions of their fuel tanks from penetrations from German shells. [Alternate translation] The gun used in the tests was a 75mm kannon 40. 
They tested three types of shells.  Model 38, AP Model 39/40 and armor burning shells.  If model 38 were the hollow charge HEAT shell it would also be classified as armor burning?  So why is it different?  I don't know.  Then what is the AP shell with red band that is fired by a Model 40 gun?  Maybe there are errors in the Russian report and can't be resolved.  Here is another Soviet test document snippet. The 75mm AP rot shell is used by the 75mm/L24.

9.   The AP cap of K. Gr rot Pz.  was softer metal and didn't permit it to penetrate sloped armor very well.  The Pz. Gr. 39 was far superior to the rot shell.  It's AP cap was blunt and made of a harder armor than the shell itself.  This allowed it to penetrate thicker armor at higher angles of impact.

10.  The  chart from DIE VORGÄNGE BEIM BESCHUß VON PANZERPLATTEN, Lilienthalgesellschaft für Luftfahrtforschung 166 (Berlin 1943) on the right shows that for gun velocities under 500 m/s the penetration of the K. Gr rot Pz. and Pz Gr 39 were virtually the same. (The muzzle velocity of the 75mm/L24 was a mere 385 m/s).  The large cavity of the K. Gr rot Pz had the drawback is that it weakened the shell such that it would break up when faced with thicker armor at higher velocity.  The penetration level would more or less level off as the velocity increased.  PDF

11.  In the Cairo tests the K. Gr rot Pz was test fired by the M2 75mm/L30 at a Panzer III with 32mm face hardened armor bolted over 30mm homogenous armor. The shell made a clean penetration of this at 1000 yds so the impact velocity was ~490m/s.  Compare this 62mm total @ 10° with the 54mm  @ 30° to the vertical that the German chart to the right says it will penetrate.
12US tests at Aberdeen after the war on the ballistics of the Pak 40 /L46

list its muzzle velocity as 790m/s.  It's penetration is greater than the L43 and L48.  So we would have to assume the US found stock piles of the early rounds still available in 1945 to test.

13Tests by the Yugoslavs in the 1950s and 1960s of various German and Allied guns show that the German Pak 40 Pz Gr. 39 had about a 5mm edge in penetration to the solid shot M79 of the US 3".  Given that both weigh the same and the 3-inch muzzle velocity is 792 m/s it is hard to see that the German round could be ~40 m/s less than the US round and still surpass it in penetration. We might conclude the Yugo PaK 40 would be 790 m/s.

14. Russians didn't seem concerned that their data doesn't go with their other data.  A firing test by PaK 40 (770m/s)on the front of the T-34 shows it penetrates at 1000m.  But this table says it only penetrates 74mm at 1000m.  46mm@60° should be more.   

15. A German test includes the 75mm/L43 (740m/s) effect on the T-34/76 finds that it will penetrate the front hull up to 800m..

But some questions remain.
1.  Did the L43 and L48 guns always use the same shell?   If so how could lengthening the barrel by only 5 calibers increase the MV by 50 m/s.?    And if it did then what did reducing the charge in the L48 do to the MV of the L43? 
One thought experiment.   The US M2 cannon 75mm/L31 with MV 564m/s when increased in length to US M3 cannon L/40 went to MV of 619 m/s.  An increase of 9 calibers increased the MV by 55 m/s.  If the same were applied to the L43 then the L48 MV would be 771 m/s.
2. Was the change from the K. Gr. rot Pz to the Pz Gr 39 done concurrent with the change to a reduced charge cartridge?  If not there would have been high and low velocity K. Gr. rot Pz. or high and low velocity Pz Gr. 39 or both.  Possibly four flavors of shells.
More data can be found on page 48.