A longer 15-inch gun offered a middle path in battleship design

Why did the Royal Navy study longer 15-inch guns rather than only repeating the 15-inch/42?

Longer 45- to 52-calibre designs promised higher performance, with a 48-calibre or greater weapon offering a more worthwhile advance over the old gun.

The 14-inch weapon normalized a 45-calibre barrel in contemporary British thinking, while a journal study also described a theoretical 15-inch/52. Improved construction meant that a new 15-inch/45 could be lighter than the older 15-inch/42, while a much longer weapon might approach the old gun's barrel weight. A 48-calibre or longer weapon would offer a clearer performance gain than a modest 15-inch/45. Calibre alone does not define a naval gun because barrel length, construction, weight, and the desired step in capability all shape the choice.