Why 12-inch guns usually displaced 11-inch weapons

What practical advantage could an 11-inch capital-ship gun offer over a 12-inch weapon?

A long-barrelled 11-inch gun might approach a 12-inch weapon at similar weight, but an equally developed 12-inch gun would usually offer better penetration and destructive effect.

An 11-inch gun could approach a 12-inch weapon by using a longer barrel at similar weight, but the comparison depends on holding the rest of the design equal. If both calibres could be lengthened, the 12-inch gun regained the advantage. Capital-ship gunnery prioritised penetrating armour and then causing damage inside the target. A larger shell therefore remained preferable when the ship could carry its barrel and mounting. Eleven inches occupied a narrow compromise space rather than being inherently useless.