Why 20-inch super-heavy shells exceed practical limits

Could a battleship practically mount multiple 20-inch guns firing super-heavy shells?

Probably not as an effective fleet weapon; metallurgy, barrel wear, propellant demands, and loading machinery make about 16.5 inches a more practical ceiling.

A 20-inch super-heavy shell demands an enormous propellant charge and exceptionally strong barrel metallurgy. A failed or incomplete discharge would create a serious hazard inside the gun, while repeated firing would punish the barrel. Every loading and ammunition-handling component must also move that mass while surviving firing shock. Ships armed above the practical limit might exist for prestige, but a useful rate of fire and manageable machinery put that limit near 16.5 inches.