A naval gun's mount can matter more than its caliber

What difference would a working British 5.1-inch/50 gun have made in the Second World War?

Its value would depend more on the available mount than on caliber alone, especially whether it could provide effective dual-purpose service on different ship sizes.

A nominally better gun changes little unless its mounting fits the mission and the ships that must carry it. The 4.7-inch weapon worked well but lacked a suitable dual-purpose mounting. The 4.5-inch system's value likewise depended on when appropriate mounts became available. A 5.1-inch gun on an unsuitable mount would not justify fleet-wide change. The complete system would have to serve carriers and smaller escorts effectively before the new caliber offered a worthwhile gain.