The US Nickle (5 Cents) ways 5 grams. I personally think that wad done on purpose as a tentative step to move to the metric system.
replies(1):
The precise five gram weight of the nickel was deliberate, but dates to the Civil War, a time when the US had no intention at all of moving to the metric system. It's rumored that a gram or two of weight was added to the coin on the premise that "five cents five grams" was a nice round number, but actually due to lobbying by moneyed interests who owned a nickel mine, so they could sell more nickel to the government.
"Uses the metric system" = uses standard metric units.
By the logic you lay out, the metric system itself doesn't use the metric system, seeing as the units are not defined by reference to SI terms but to natural constants. (And if your definition of the metric system includes both US customary units and natural constants, then your use of the phrase 'metric system' has ceased to signify anything meaningful.)
Of course the metric system doesn't use the metric system. How would that even work?
SI units are, as you point out, defined in terms of measured aspects of reality.
US customary units, on the other hand, are defined in terms of the metric system.
And yes, this means that both of them are grounded in measured aspects of reality. Again. How else could this possibly work? Measuring themselves, perhaps?
If US customary units were defined directly in terms of measurements, then they wouldn't use the metric system. They would only be interconvertible through those measurements, and if they weren't the same measurements, as they easily could not be, this would be an empirical process subject to further experiment and refinement.
But they're not. They're defined in terms of SI, making conversion a matter of arithmetic.
It's a load-bearing statement. I do understand how someone who lacks grounding in the physical sciences might not realize that. Glad to help.
Condescending and wrong, and all in defence of the facially farcical statement that the US uses the metric system. Not that US customary units are defined in terms of metric quantities, which is the moved goalpost you're now trying to defend, but that "the US uses the metric system".
Whether or not the US 'uses the metric system' is not contingent on what units US customary units are legally defined in reference to, that's merely trivia.* US customary units do not form part of the metric system, and metric units do not enjoy widespread use in the United States (with minor exceptions).
Ergo, the US does not use the metric system.
* And relatively recent trivia at that, US units were naturally not first defined in metric units, only later restated as such.