Why modern naval guns rarely grow beyond eight inches

Would widespread six-inch guns lead navies back to eight-, ten-, or eighteen-inch weapons?

Six- and perhaps eight-inch systems can be justified, but larger weapons create major integration problems and usually lose the practical comparison with missiles.

Modern six-inch weapons still have plausible roles, as might an eight-inch railgun. Beyond that, mount size, handling, and integration costs rise rapidly. A navy must then explain why a larger gun is preferable to a missile that can deliver range and effect without the same turret burden. Whole-ship burden and alternative weapons set the practical threshold more firmly than the engineering ability to enlarge a bore.