The successor to the L/45 gun employed in the North Carolina and South Dakota classes. That gun had been designed before the development of the super-heavy 406mm shell (which in effect was better than the 18" shell of the IJN), and its 45-caliber long barrel could not speed the shell to the designed muzzle velocity. By lengthening the barrel by five calibers, the new heavy shell could be fired with the same velocity of a regular shell, making the L/50 gun more than equal to the 18" gun of Yamato and Musashi.

    The gun that had originally been supposed to be used in the Iowa was a L/50, 406mm gun too, but one designed in 1916(!) for the cancelled new battleships of the South Dakota (BB-49) class. However, their design was too heavy for a new ship, and a "light" gun was developed. 

    Problems with the accuracy of this gun were encountered, though, as its allignment was poor at first. These problems were corrected after the Battle of Leyte Gulf. What their effect on actual combat performance was is unknown, but it should be noted that in the one engagement they fought, during the raid against Truk in February 1944, their performance seems to have been well and no difficulties were noted.

    The same gun would have been employed in the Montana class, had it not been cancelled.