The treaty logic behind a 14-inch battleship gun

Why did Britain and the United States consider a 14-inch battleship-gun limit?

They hoped common calibre and tonnage limits would preserve the battleship, with Britain expecting its designers to gain an advantage within equal rules.

Chatfield and other supporters of smaller guns hoped a common 14-inch limit would make battleships more survivable and affordable. If every navy worked under the same maximum tonnage and calibre, the ships would begin from broadly similar constraints. British designers expected their experience to turn those common rules into an advantage. The proposal therefore joined arms limitation to a competitive belief in superior design rather than simply seeking weaker ships.