Or 10mm if you’ve wrenched a car

People know what the bolt looks like, but the wrench is missing.

ahh fellow knuckle buster too

Yes, but how many of you reflect over the nomenclature? How many of you know the actual diameter of a "twenty-two" (even if spelled out as .22)? People don't often think about the meaning behind colloquialisms.

American public schools have been using metric for decades. Science classes in American public schools use metric exclusively since the 80s at least. When you meet an American who makes a big deal out of not knowing what a meter is, 9 times out of 10 they're putting on a performative patriotic display mean to get a rise out of you. That, or they're quite elderly.

Incidentally, .22 LR is nominally but not actually 0.22 inches in diameter. It's very slightly bigger than that. In the domain of ammunition, there is often a mismatch between actual diameter and nominal diameter.

With calibers there is a difference between american .22 and european .22. As I understand it, the american measurement is slightly larger as it includes a bit of the rifling groves whereas the European standard was only the bullet width. So the same bullet measures slightly larger on the american scale. Or I might have that backwards, one of the two is different.

https://www.rifleshootermag.com/editorial/comparing-american...

From what I understand, ".22 LR" is nominally .22 inches, actually is .223 inches, and is labeled as 5.6x15mmR in Europe while actually being something like 5.66mm (the 'x15' is the length of the case in millimeters, and the R means rimmed.) They are called different things and both are not quite their nominal sizes, but I think they're interchangeable in practice.

It looks like .38SP!