Machine guns that could damage fragile 1920s aircraft became inadequate as aircraft grew larger and tougher. Destroyers received 20 mm and 40 mm weapons, including the 40 mm pom-pom and Bofors systems. By the end of World War II, the discussion places 3-inch guns in the light-to-medium anti-aircraft role. The comparison runs from 12.7 mm in the 1920s to 76 mm in the late 1940s and early 1950s, roughly six times the calibre for broadly the same defensive function. The appeal of the 3-inch gun was its combination of shell size and a very rapid rate of fire. It could concentrate substantial flak within an area.
Notable Point · 48:33-50:42
Aircraft growth drives larger destroyer anti-aircraft guns
The defensive role assigned to 12.7 mm machine guns in the 1920s eventually calls for 76 mm weapons, combining a larger shell with rapid fire.