Japanese small arms of WW II was a mess; crappy weapons,and different calibers.
My Father-in-Law gave me his WWII bring back type 14 Nambu. It was beautifully made and accurate. It had the best trigger I have ever seen on a military sidearm.
Their pre-war rifles were also fine firearms. As the war drug on, the quality declined.
I do agree with the odd number types of ammo. I think there were several different types of their .31 caliber ammo. Some for one type of machine gun, some for another and some for rifles.
I am a particular fan of their Type 89 50mm Grenade Projector ("Knee Mortar"): fired a 1 1/4 pound spin-stabilized round with almost a pound of TNT filler and excellent fragmentation out to 325 meters accurately. According to some sources, accounted for nearly 60% of our Pacific War casualties. Darn thing could also fire their standard hand grenade by adding a screw-on booster out to 150m. We need a new version of that thing right now.