The available 4.5-inch weapon used a between-decks mounting that would penetrate the deck and force major internal rearrangement. The 4.7-inch alternative can sit above deck, preserving far more of the existing structure. Converting a six-inch magazine to hold smaller 4.7-inch ammunition is comparatively straightforward because the storage fittings can be altered without rebuilding the ship around a new gunhouse. Installation geometry and conversion time can therefore matter more than choosing the theoretically best gun in isolation.
Notable Point · 1:02:01-1:02:53
Why the rebuild favors a 4.7-inch upper-deck mounting
The 4.7-inch weapon is chosen over the 4.5-inch gun because its upper-deck mounting avoids an expensive internal reconstruction of the old cruiser hull.