The 127mm L/25 gun was employed in the anti-air (secondary) batteries of all pre-war cruisers save Wichita, St. Louis and Helena. It was the 5" L/38's direct predecessor, and like it, played an important role in the war. 
It originated from a 1921 requirement of a heavy AA gun to replace and supplement the older 76mm gun in use aboard the fleet's battleships and cruisers, which did not originally have AA fire as prime purpose. Qualities sought were good handling, quick rate of fire, and a good explosive effect. 

    For quick rate of fire, a fixed round was desired, for good handling, a short barrel that would yet sufficently speed the shell was asked for, and for good explosive effect, a large caliber was sought. All these factors added up to the 127mm L/25, a hand-moved gun until the 1930's, which was quite simple in design but a good gun.

    Its high rate of fire made it a candidate for surface fire as well, and it was adopted in favor of the L/51 gun aboard the new treaty cruisers, which would have relied on the L/51 for illumination rounds and now would use the L/25 in a dual role. Four were originally mounted, but with the removal of the torpedo tubes in the mid-1930's, four more were added, giving the heavy cruisers their only capable anti-destroyer weapons. 

    The gun was mounted on battleships from Nevada to Maryland, on the above mentioned cruisers, on fleet auxilliaries and merchant ships. Slightly different guns of the same type were mounted on fleet submarines. These were wet mounts, requiring no protection at all during dives, and being very well liked with the submarine force that had had to make do with 76mm and 104mm guns before. 

    All pre-war heavy cruisers, most light cruisers, and three battleships retained these guns until the end of the war.