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

because they’re just names and people defined things differently. sometimes different calibers use a different diameter in the name despite being the same actual dimension, just to avoid confusion. there is no central regulating body for caliber naming and there certainly wasn’t 100+ years ago