Why a supergun race would reverse toward smaller weapons

After G3 and N3, competition might briefly produce 20- or 21-inch guns, but poor rate of fire and ranging would push navies back toward more numerous 16.5- or 18-inch weapons on fast battleships.

Question

Why would twelve 18-inch guns not become the permanent standard after G3 and N3?

Short answer

A brief calibre race could reach 20 or 21 inches, but testing would reveal poor firing and ranging performance and drive designers back toward denser 16.5- or 18-inch batteries.

Notes

An 18-inch British ship could prompt a larger American gun and then a 21-inch British reply. Trials would expose the operational cost of slow loading and sparse salvos. Designers might use the fast-battleship or fully armoured battlecruiser label to justify returning to a smaller calibre with more barrels. A twelve-gun 16.5-inch ship would offer better ranging and rate of fire. Later estimates put such a fully armoured design around 84,000 tons, so even the more efficient option remains an enormous industrial commitment.