Japan and Germany are entirely metric. Pretty much the entire US manufacturing industry, or rather the zombie the pentagon keeps on life support, is in US standard.
That means everything needs to be adjusted or replaced. That's not viable in peace times, it's even less so in times of war.
Obviously this applies primarily to tanks and ships, not planes nor guns.
Part of the beauty of eg. CNC mills is they don't necessarily care what the units are.
Especially when inch is now defined as being exactly 25.4 mm and you can manufacture to 0.01mm with commodity-level hardware.