Many Japanese cruisers carried 6 inch main armament leading up to and during WW2. Several pagoda battleships had similar bore secondary armament. That many of the IJN naval rifles resembled RN weapons (e.g. Vickers) is no coincidence, as British weapon design and naval construction had a great influence in early Japanese naval systems around the turn of the twentieth century.

FlyHawk Model has carefully produced a 6 inch barrel to improve the look of medium bore guns in 1/700th scale. These are beautiful tapered brass gun barrels that will immediately enhance the appearance of your ship’s weapons. You get 10 barrels in this set. With such broad application in service, FlyHawk’s FH 700074 metal barrels are the perfect upgrade to replace those ugly plastic barrels.

FlyHawk keeps providing us with 1/700th scale specialty items to replace the details too delicate to reproduce in plastic. Thanks FlyHawk!

References:
There is plenty of information out there about IJN naval artillery. A good place to start if you like to do your research online is Navweaps' Japanese Naval Guns. Otherwise, everyone’s “go to” source is Warships of the Imperial Japanese Navy, 1869-1945, by Jentschura and Jung.