The ship-design cost of a Japanese 20-inch gun

Were reports of a Japanese 20-inch naval gun credible?

Possibly, but making the weapon work would demand an exceptionally large ship and difficult solutions for weight and stability.

Reports of Japanese work on a 20-inch naval gun are plausible, although a practical weapon might never have resulted. The mounting and ammunition would impose enormous weight high in the ship. Moving the turret could create stability problems serious enough to require compensating ballast. A workable platform might need to exceed even the Yamato class, multiplying the industrial and naval-architecture costs of the gun. Firing a weapon of this scale could also damage the battleship's own electrical systems and other internal equipment.