Why 16.5-inch guns rarely justify replacing 16-inch guns

What is the real difference between 16-inch and 16.5-inch naval guns?

The larger gun brings modest gains in shell size and range. It is more attractive when moving up from 14 or 15 inches than when replacing an established 16-inch system.

A 16.5-inch gun offers a somewhat larger shell and longer range than a 16-inch gun. The improvement is too small to justify replacing an established 16-inch system. A navy moving up from 14-inch or 15-inch guns has a stronger reason to choose 16.5 inches because it is already changing calibre. Once either size is in service, familiarity and logistical continuity favour retaining it. The same preference for an established calibre is applied to battleship design in a computer game.