46 cm/45 Type 94 naval gun
The Japanese 46 cm/45 Type 94 naval gun was a 46 cm (18.1 in) naval gun with the largest bore diameter of any gun ever mounted on a warship. Only two ships carried them, the Imperial Japanese Navy's World War II battleships Yamato and Musashi.
Source: Wikipedia — 46 cm/45 Type 94 naval gun (CC BY-SA 4.0)