Secondary armament has to justify its weight through a mission that the main battery does not already perform. If a battlecruiser cannot be protected for a close-range fight against the newest capital-ship guns, six-inch secondaries add little to that impossible contest. Destroyers and smaller attacking craft remain threats that the ship can and must defeat. Renown and Repulse therefore illustrate why a heavier 15-inch main battery could be paired with a return to more numerous four-inch anti-destroyer guns.
Notable Point · 37:19-38:08
Secondary batteries should follow the threat
Once armour cannot defeat the heaviest opposing gun, the choice between six-inch and four-inch secondaries should follow the destroyer and small-craft threat.