What drove the move from adapting existing guns to designing guns specifically for anti-aircraft work?
Short answerHigh elevation, faster training and mounting weight forced designers to treat gun and mount as one system.
NotesAn anti-aircraft gun has to work in a mounting capable of high-angle fire. Its training speed must keep up with aircraft movement while accommodating the ship's motion and weather. As mounting weight becomes a constraint, the choice of gun and barrel also has to change. These requirements jointly pushed development towards guns designed specifically for anti-aircraft work.