Moving from 30 mm to 40 mm adds more than nominal calibre. The larger shell has enough internal volume to accommodate newer ammunition features without forcing the same cost and engineering compromises found in smaller rounds. That space gives the weapon a more practical growth path as ammunition technology develops. The argument supports a modest patrol-ship upgrade while keeping the weapon below the cost and complexity of 57 mm or 76 mm systems.
Notable Point · 2:59:29-3:01:26
Why 40 mm leaves room for better ammunition
The extra volume inside a 40 mm shell makes advanced ammunition easier and more economical to develop than comparable 30 mm rounds.