Dan K wrote:
The IJN used metric system measurements.
Yes... but there are some quirks to that, such as the 14" battleship guns that were called 36cm, though they really were true 14" diameters; old construction, a holdover from their British origin true... but then there's their WWII heavy cruiser guns which were officially designated as a 20cm/50 weapon, and sometimes a bore diameter of 20.3cm is stated, but were in fact 20.32cm, or precisely 8.0" to max out the gun size to treaty limits. The Akagi's 12cm weapons followed a series of earlier 4.7" guns that were redesignated as 12cm after Japan went metric, and if they were able to keep firing the same types of projectiles then they probably matched the diameters of the earlier guns.
- Sean F.