They most generally use the centerline of the bore when measuring the rifle stock dimentions. It just makes for a common starting point regardless of what type or mfg sites or scope mounts are on the rifle.
I've seen a couple of builders use a 1" dia bar thru the scope rings to extend back over the butt stock and take dimentions off of that.
A much easier measurment to relate to for some and the best way to fit up a custom rifle stock if that is the way the rifle will be set up for the customer.
But it's not a quick and easy thing to do,,taking the rings and scope off, and exchanging them for the bar,,taking measurements and then replacing the scope... if all you want is specs to give to a potential buyer. The centerline of the bore is what it is. Scope ring heights can be all over the place.
The dimentions can actually be taken from any of the points,,off a rib,,off the sites,,scope bases,,scope rings,,etc. As long as the person receiving the specs understands where and how they were measured, there shouldn't be any misunderstanding.
...Nice 21..
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