Naval guns for a long war

What main gun system is most useful for a modern warship?

The OTO Melara 127/64 is the preferred current system, with a potential role for a 155 mm naval gun in a long conflict where missiles cannot be assumed to last.

The OTO Melara 127/64 is the preferred mature and capable modern naval gun. A 155 mm naval weapon could also draw on the manufacturer's land-artillery experience to provide heavier sustained fire. The strategic case extends beyond calibre preference: missiles and their production base are finite, while a prolonged conventional war demands weapons that can keep firing. A fleet designed only for a short conflict fails by default if the war endures, whereas long-war preparation still works when a crisis ends quickly.