British capital-ship guns had already advanced from 12 inches through 13.5 and 15 inches in successive construction cycles. Royal Navy practice favored stepping up calibre rather than adopting triple turrets, and surviving work on 16.5-inch and 18-inch weapons shows the next choices already under study. A 16.5-inch weapon could preserve roughly the rate of fire of a 16-inch gun, while 18-inch and proposed 21-inch weapons traded firing opportunities for greater impact and prestige. The practical core of the fleet could therefore use 16.5- and 18-inch guns while a few 21-inch ships answered an American effort to overtake British calibre. The result is a fleet shaped by both gunnery efficiency and the political desire to field the world's largest weapon. An Ultimate Admiral: Dreadnoughts illustration places eight 18-inch guns on a Queen Elizabeth-derived ship of roughly 50,000 tons; this is a game reconstruction. Fisher's inquiries to manufacturers in 1914-15 also produced discussion of guns as large as 24 inches, describing possible manufacturing capability rather than an approved ship program.
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Britain's projected 18-inch and 21-inch capital ships
A prestige-driven gun race could produce 18-inch fleet ships and limited 21-inch flagships, even if 16.5-inch weapons offered a better rate-of-fire compromise.