Thanks to Babykim, EU
Some explanations about the penetration mechanic in relationship to the changes for German guns by developer BadPreacher.
Let us start with HE. The penetration is computed according to the 1/6 or 1/4 caliber rules, rounding to the nearest whole number. The resulting number is the armor thickness that the HE shell cannot penetrate. For example, for a 152mm HE shell we get approximately 25.334 (152/6), which rounds to 25, meaning that 25mm will not be penetrated, but 24mm or thinner will be.
The 1/4 rule in 0.6.6 for German guns applies as follows:
1. Main battery: all battleships and cruisers of tiers 4-10,
2. Secondaries of 128mm and 150mm caliber: all battleships of tiers 7-10, and cruisers Roon and Graf Spee.
Turning to the AP. The penetration is computed according to the 14.3 rule, without rounding. Divide the gun caliber by 14.3. A bounce cannot occur if the result exceeds the thickness of the armor. The check for penetration accounts for the impact angle (normalization), so that the normalized line-of-sight thickness* of amour applies. Consequently, even if the shell did not bounce it may fail to penetrate the armor.
*) Quote Wikipedia: For example, armour sloped sixty degrees back from the vertical presents to a projectile travelling horizontally a line-of-sight thickness twice the armour’s normal thickness.