Funny

Nah, we use decimal inches too.

If we use metric, it's because the origin of the round or gun was specified that way in it's country of origin.

Plus with ammo, specifying the caliber in it's original spec of metric or ACU also implies other attributes. For example, 5.56 /=. 223. While both rounds have the same nominal diameter, the shape of the casing is different, and the max pressure specification is different. I forget which way it works, but one round can be used in the other gun, but not the reverse.

Then there's things like the .308 round, which people often equate with a 7.62 even though it's actually 7.82mm. These two rounds are also only interchangeable in one direction (and it's still not recommended).

Note: Don't hold me to these exact measurements and the differences. I'm going from memory so may have things reversed. But you get the idea: the numbers reference two different approaches to how ammo is measured and designed. It's not just using different numbers, these are different labels indicating different specifications.