Why are pistol calibers measured with the bullet diameter, but bigger calibers the bore’s land diameter?

Question

E.g. The bullet of 9×19 is 9mm wide but of a 5.56 isn’t 5.56

Edit: 5.56 not 55.6

In: Technology

Answer 1

Everyone uses a mix of ammo calibers.

My very American pistol fires 9x19mm which was developed by an Austrian man named George Luger

But 9mm is a good caliber so it’s now a standard ammo type world wide.

This can be applied to almost every ammunition caliber in the world, some are great and you can probably list them off.

Others where one off ammo types used rarely. Like 8mm Mauser. It’s not used anymore really. But was big in ww1.