[Suhiir]

Ammunition is a trade off between diameter/mass, powder charge/velocity, and if it has a warhead bursting charge.

Take the classic American .45 caliber M1911, big low velocity bullet and limited magazine capacity because of this. This is why many armies use 9mm handguns. Smaller higher velocity bullet and a much larger magazine capacity. Which is "better"? That's an argument that will never be resolved because it's more subjective then objective.

"Odd" calibers are because of this. You trade off caliber for mass, ammo capacity, and often velocity. Sure they make big high velocity ammo, but due to it's size it's HEAVY. This is why naval guns are generally larger then land based systems. You have a whole ship to store ammo and ammo handling equipment in.