The newer U.S. battleships that fought in World War II had many important gunnery advantages over earlier battleships, such as excellent fire-control systems and, by 1944, the best surface fire-control radar in the world. They also had a hidden advantage in their magazines: rather unusual shells.
When considering the characteristics of battleship heavy guns, normally what first comes to mind is caliber (bore diameter) and muzzle velocity (projectile speed leaving the barrel). You could also consider fire control. You probably wouldn’t go a step further to look at shells, except for noticing that some were a lot better than others at piercing armor (such as at Jutland in 1916).
The interwar U.S. Navy Bureau of Ordnance went the extra step. In the early 1930s it began to ask whether heavier armor-piercing (AP) shells would be worthwhile. Like most other naval developments of this period, the heavy-shell idea was tied up with naval arms limitation.