Why the early 4.5-inch mount was difficult for destroyers

Upper-deck and between-deck mountings differed in layout and readiness: HMS Ark Royal received the upper-deck version because the between-deck design was not yet ready, while later carriers and modernized capital ships used the latter. The early 4.5-inch upper-deck mounting was designed more naturally as a carrier or cruiser secondary battery than as a destroyer's main armament. Its ammunition and manual handling imposed a heavy physical burden, particularly if one-piece rounds were expected to sustain a useful rate of fire. Adding enough relief loaders would enlarge the gun crew without solving the underlying endurance problem in a sustained action. A workable destroyer installation would therefore need two-piece ammunition or meaningful loading assistance rather than simply transplanting the existing mount.