How Do You Measure a Group?

[Surculus Solitudo]

I recently sighted in my .243 rifle.  I have always measured my groups from center to center on the largest spread between two.  I am sure that I do it this way because that is the way my Dad did it.  It got me to thinking though.  Should measure from outer edge to outer edge. Or from inner edge to inner edge. 

Which method do you think is correct?

[papajohn]

I think it's easier to measure the outer edges of the two widest holes, then subtract bullet diameter.  Finding the center of a hole in paper can be pretty subjective, but finding the edge of a hole is hard to screw up.

[Surculus Solitudo]

Thanks papajohn.  That sums it up nicely.

[gearheadpyro]

You shouldn't subtract the bullet diameter as that will give you an incorrect (but only slightly) result. When a bullet goes through paper the hole is smaller than the bullet diamter due to the paper stretching slightly. In my .308 shooting into card stock the difference is .014"

Measure the diameter a the bullet hole in the paper, then measure outside to outside of the group and subtract 1 bullet hole.