Hi again,
the problem with the concrete fortifications is not to how to penetrate thm but how o accurately target them. The Italian Navy had major dispersion problems with all guns of 100 mm and above. These problems were never satisfactorily solved, despite all kinds of tests ad modifications.
Penetration data extrapolated from muzzle velocity, mass and shell composition would indicate the 381 mm APC could penetrate in excess of 550 mm steel armor (vertical) @ 15.000 m range, whereas the 320 mm guns could pierce roughly 30 to 40% less (remember these guns were rebuilt 305/46, their balistic performance did not greatly improve over the original). Therefore it would have been necessary to achieve repeated its in one spot of a concrete bunker or similar building to obtain penetration, and that was clearly outside the capabilities of the gunlayer, unless the range was pointblank - or the concrete not properly cured.
At our disposal for further details,
regards, Brian