An 18-inch weapon is not simply a slightly wider 15-inch gun. At the same 50-calibre proportion, the barrel alone is roughly twelve feet longer than its 15-inch counterpart. Its larger chamber and heavier handling mechanisms then demand a much larger turret and barbette arrangement. The visual comparison makes the design penalty behind very-large-calibre battleship guns concrete.
Notable Point · 23:15-26:01
Why 18-inch guns need much larger turrets
Comparing 15-inch and 18-inch guns explains why the jump in calibre reshapes barrels, chambers, loading gear, and turret layout.