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

Its just 8.82 gets rounded up to 9. It its entirely in line with the “land diameter X case length, round up/down” naming convention thats been in use for over a century.