Reading the Type 31 as a flak frigate

Why choose a 57 mm gun for Type 31 rather than a larger 76 mm weapon?

Because the 57 mm better fits a heavy-flak role, while 76 mm is the stronger general-purpose choice.

The intended target matters more than calibre in isolation. A 76 mm gun offers a broader light general-purpose role, while 57 mm is suited to heavy flak. Type 31 combines 57 mm and 40 mm weapons to build layers of close-range anti-aircraft fire. The aim is to break an incoming threat into less damaging fragments or destroy it before impact without spending a missile on every engagement.