Thx for the explanation!
But I still don't understand this:
In each case I fire AP at a still standing Furutaka @ 4km.
I always aim at the same area.
130mm AP (33.5 kg) - Boom - 13 citadel hits, target destroyed in 22 seconds.
152mm AP (47.6 kg) - Boom - 6 citadel hits, target destroyed in 22 seconds.
356mm AP (673.5 kg) - ...... - 0 citadel hits, target destroyed in 61 seconds.
Why do 130 / 152mm AP rounds still wreck the Furutakas citadel but 356mm AP does NOT?
Can you redo the test with Fuso shooting the Furutala closer to the waterline? Maybe the BB shells riccochet off the citadel roof as mariouus stated, so aiming lower should mean hitting the side of the citadel.
Another assumption is that BB shells simply overpenetrate cruisers citadels, though i don't know if this is possible.