As From here the fixed table with all 3 Ammo Types
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| Stat | 3cm M.K | 5cm KwK |
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|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.