Come on, Keith! If you have a barrel with an EVEN number of lands and grooves, you have lands that are OPPOSITE each other.
On the other hand, if you have a barrel with an ODD number of lands and grooves, you will NEVER have lands that are opposite each other!
Because MOST modern barrel makers want NO run-out, they either plumb bore the barrels, and put the flats on after the bore is drilled, or regrind the flats to match the line of the bore After boring an octagon barrel.
Either way, for barrels with Odd Numbers of lands and grooves, you can measure the bore diameter by measuring the thickness of the barrel from an outside flat to the nearest land. Multiply that measurement by 2, to get an "artificial" idea of the barrel's thickness on opposite sides, and then subtract that sum from the width of the barrel ATFs. You may not get the Actual bore diameter within 10,000ths of an inch, but you should be within .001" of actual dimension. AND, I concede that this is not easily done if the outside of the barrel is round, rather than octagon. :shocked2: :surrender:
Most machine shops have barrel gauges, and can measure your bore diameter for you usually for a small cost, but sometimes at no cost to you at all. They consider this small kind of service a "Loss leader" ( in sales parlance) that will have you either taking other "real" business" to them in the future, or referring friends to the business.
You also can "slug" a barrel, and use outside micrometers, and barrel gauges to measure the bore diameter off the soft lead slug that emerges from your barrel.
