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

It has mostly to do with origins. US uses inches, so American calibers are in inches. Most of the rest of the world uses metric, so their calibers are metric. 9×19 is European, 5.7 is European. .223 is American, but NATO is mostly European and adopted the 5.56.