Measurement of calibre in GM barrel?

[Jarikeen]

Can anyone please let me know where the calibre of a green mountain ML barrel is measured from? - is it measured from the lands, the grooves, or half way between?
Cheers

 

[Le Grand]

Yes, the calibers are taken from the "bore diameter." That's what you called the diameter from land to land. "Groove diameter" is measured from groove to groove. A .50 caliber barrel, for instance, will generally have a bore diameter of .500" and a groove diameter of .524" if the grooves are .012" deep. Le Grand

 

[CrackStock]

The caliber is the size of a ball that would touch the outer edges of the lands.

Sometimes this is hard to measure since some barrels have an odd number of lands, so you cannot just measure across with calipers.

CS