I have started using the Marder A1 and the poor penetration of the 20mm barrel seemed quite strange to me, especially using tungsten carbide bullets and being bullets from the 60s.
First I asked a colleague who has created a suggestion to change the horrible formula for calculating penetrations for a better one, and in the calculation of the 20mm DM43 ammunition it gave a penetration of 91mm at 0º and 0 meters, then I found some images on several sites.
In this image there are several penetrations of 20 and 30mm ammunition. In the one referring to the DM43 it is seen that it penetrates 32mm at 0º at 1000 meters, while in the game the penetration is 24mm.
In these two images you can see what appears to be a German test of DM43 shots against the rear part of the turret of a T-62.The shots are at 100 meters, and the DM43 is capable of completely piercing the rear part of the T-62 turret, hitting the flattest part, and almost penetrating the areas with the most angles.
As you can see, in the game he is not able to pierce it.
It would be great if gaijin finally decided to update the penetration calculator, since there are many bullets that would achieve minor changes, but the APCR would be greatly improved, which are one of the most poorly treated bullets in the game since it came out, and which in many cases are bullets that you never use because of their extreme ineffectiveness.