Why boring out a 410 mm gun sacrifices barrel margin

How far could the Japanese 410 mm Year 3 naval gun be bored out?

Perhaps to roughly 420-425 mm, but only with important penalties in barrel strength, rifling, life, range, and accuracy.

Boring out a gun enlarges the projectile but removes material that supports the barrel and its rifling. The result also becomes a shorter-caliber weapon in proportional terms, affecting range and accuracy, while the Italian experience warns of accelerated barrel wear. A cautious estimate is roughly 420 to 425 mm rather than a dramatic jump in caliber.