JOC wrote a good chapter on it in the Rifle Book titled "how Cartridges are Named" I think. Some for bore diameter and some for groove diameter. I've never understood the rant about metric. metric is what it is. We here in the US have grown up with the imperial system so we're comfortable with it. Europe has the metric system and our autos are a mix of the two. You can convert and machine just as precise either way. Threading metric with an inch lead screw lathe that's geared for it isn't as easy as cutting imperial threads. Cutting metric threads on a metric geared lathe isn't so easy either. The thread dial is a mass of gears. Then there's British Whitworth.
