One explanation offered for the Royal Navy's loss of interest in 3-inch guns was that the 4.5-inch weapon appeared too close in size to justify carrying both. Moving the main gun to 5 inches could make the smaller weapon easier to justify as a separate system. This is a reported procurement argument about separating weapon roles. A further move to 155 mm is suggested as a personal preference, with the acknowledgment that the proposal had found no support.
Notable Point · 1:26:01-1:26:37
How larger main guns could make room for 3-inch weapons
A reported procurement argument connects the survival of smaller naval guns to how clearly their roles differ from a ship's main gun.