Why 18 to 20 inches is a practical ceiling for naval guns

Is there a maximum practical caliber for a naval gun?

Roughly 18 inches is practical and 20 inches is possible, but barrel size, shell handling, armor, and propulsion make larger guns increasingly unattractive.

Eighteen inches sits near the practical limit, while 20 inches remains possible only with serious design penalties. A 20-inch, 45-caliber barrel would be about 22.5 meters long, before accounting for the mounting and machinery needed to handle it. A ship built around such weapons must also decide whether to carry armor for close action or rely on range, speed, and machinery power instead. The result is a rapidly worsening exchange between gun size and the rest of the battleship rather than one hard ceiling.