Not interested in winning the cartridge of my choice, but for someone who grew up buying a quart of milk at the market so the problem is solved. Still it’s easier (to visualize a quart than a liter of milk but It’s all how one relates. Now most everything is packaged in metric sizes at the markets, And this “1 dm^3 = 1 liter” all I have to say is, HUH? is that Greek? But I also have a hard time dividing a gallon of milk into cubes, too much spillage.
To again take John Kindred’s point I can easily visualize 1/32 of an inch but tell me .79375mm or even .8mm & I’m totally lost, so how many hairs is that?. But then in a couple of weeks I’ll be 76,
And as the ammunition, if it’s a US arsenal made Cal. .30 Model of 1906 it’s a .30-06. if it was made by DWM and headstamped 7.62x63 then it’s a 7.62x63.
Goes back to my above point of history and the maker.
This brings up how does one describe the difference in a Cal. .30 Model of 1901 vs a Cal. .30 Model of 1903, both measure the same if one goes metric.
Now with the .223 & the 5.56x45 and the same with the .308 & the 7.62x51 having in some / most non-military firearms having a chamber that will safely accept both, the line if even further blurred.
I know this is about 20mm case nomenclature, but it is going into these smaller case types, and it does sound to me like Tony W. does have this 20mm problem sussed.