How World War II layered air defense was meant to work

The doctrine was layered rather than a simple competition between heavy and light anti-aircraft guns. Fighters were expected to fragment a raid into smaller groups. Heavy dual-purpose guns then used blast and explosive effect to break those groups into individual attackers, which close-range weapons could try to destroy before weapon release. Replacing too many five-inch or four-inch mounts with 40 mm guns would remove the middle layer and leave the close defenses facing several aircraft at once.