As From here the fixed table with all 3 Ammo Types

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| Stat                  | 3cm M.K  | 5cm KwK  |
-----------------------------------------------
|Penetration(AP/APCR/HE)| 95/110/15| 67/130/25|
|Damage(AP/APCR/HE)     | 30/30/40 | 70/70/90 |
|RoF (RpM)              |   0.22   |    24    |
|Aiming Time            |   0.22   |    2.3   |
|Weight                 |    141   |    700   |
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I conclude that people choose the 3cm with AP because it has a better Armor penetration, which could be more useful in more situations as a low penetration HE shoot.

As far as i can read, the 3cm is also a auto-cannon which could be a higher burst damage and/or a better performance when shooted while driving.

As seen here; note the fixed table with all 3 Ammo Types:

-----------------------------------------------
| Stat                  | 3cm M.K  | 5cm KwK  |
-----------------------------------------------
|Penetration(AP/APCR/HE)| 95/110/15| 67/130/25|
|Damage(AP/APCR/HE)     | 30/30/40 | 70/70/90 |
|RoF (RpM)              |   0.22   |    24    |
|Aiming Time            |   0.22   |    2.3   |
|Weight                 |    141   |    700   |
----------------------------------------------

I conclude that people choose the 3cm with AP because it has a better Armor penetration; which could potentially be more useful in a wider variety of situations than a low penetration HE shot.

As far as i can read, the 3cm is also an auto-cannon, which could provide higher burst damage and/or better performance when shot while driving.