Measuring center to center is more accurate because how else would you compare say a .338 and 7mm mag' s? The 7mm dia. would always out shoot the larger .338 dia. caliber.
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" Lord, make me fast and accurate."
the correct measurement is center to center........but if you really want to get precise, use a circle templateto measure the center to center group.....it' s a better way.
The easiest way is to measure outside to outside of the farthest two holes and subtract one bullet diameter.It is the only accurate way in the case of those one large hole groups.
Outside of heavy barrel rifles, I seldom see three to five shots all overlapping (despite what some folks on the net claim!) But you' re right stubblejumper, the few times I actually take the time to measure my heavy barrels, I do have to use the " subtract the bullet diameter" method. I wasn' t even thinking about that.
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