Gun caliber, barrel pools, and fleet logistics

British and American gun programs advanced in alternating caliber jumps, and a meaningful qualitative leap needed to be large enough to matter. The British 15-inch/42 proved a reliable weapon even if it entered service before a longer 45-caliber version was ready. Its value grew because many battleships, battlecruisers, and later monitors could draw on the same ammunition and replacement-barrel pool. That commonality supports a counterfactual preference for the F3 battlecruiser design's 15-inch/50 armament over the N3 battleship design's 18-inch guns, because F3 weapons could have modernized the wider 15-inch fleet within treaty limits. Common equipment reduced fleet-wide logistical friction and made it easier to rotate worn guns among ships, unlike a separate caliber used by only a few hulls.