Not having a real answer i ran the measurements and this is what i came up with. Hope you can follow what i am saying.
Rifles using standard measurement (ie..foot,inch,ect) get caliber from groove diameter.
Rifles using metric measurement get caliber from top of land diameter and have a standard groove depth for easy uniformity.
WHAT THEY ARE / WHAT THEY SAY
.223=5.66 ..................5.56MM
.243=6.18 ..................6.00MM
.264-6.71 ..................6.5MM
.308-7.84 ..................7.62MM
.323=8.21 ..................8.0MM
DISCLAIMER
Remember this is just what i figured out on my own and has absolutely no bases in any provable fact that i can find nor did i do more than a passing bit of research on the subject.
Gary
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