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

There’s actually no such universal convention.

Classic examples:
9x19mm (aka 9mm Parabellum) has a *bullet* diameter of 9.01mm and a land diameter of 8.82mm
9x18mm (aka 9mm Makarov) has a *land* diameter of 9.00mm and a bullet diameter of 9.25mm
Both are pistol rounds.

It depends much more on the local custom at the time a round is created and as other people already mentioned, sometimes there are additional reasons like trying to avoid confusion with similar cartridges or basing it on other dimensions found e.g. on the case that lead to *neither* of those two measurements to be reflected in the caliber name. (.38 Special says hi! .357″ bullet and .346″ lands)