Why not fit 4.5-inch guns?

Why not use 4.5-inch guns for the rebuilt Town-class cruisers?

The available 4.5-inch mountings penetrate the deck and are already needed for carriers and capital-ship refits. Abundant destroyer 4.7-inch guns avoid those competing demands.

The 4.5-inch gun is accepted as the better weapon for the proposal, but its available mounting makes it difficult to adopt. A between-deck installation would require extensive changes to the hull and superstructure. Four mounts, two forward and two aft, were considered for the cruiser. Obtaining them would take weapons away from carrier construction or capital-ship modernisation, so the design uses 4.7-inch guns already in quantity production for destroyers.