Paul,
I got into a argument one time and measured it from center to center and got a whopping .009 differance, so why is everyone making such a big deal . So for posting on the board I think measuring from out side to out side and subtracting diameter of the bullet is good enough. And that is all I will be doing and if everyone wants to flame we that ok with me .
I don't think there is a "big deal", until and unless a contest is at stake. Then a target fixture, like those used at a BR match (I don't have one or I'd post a picture) will get down to .000" center to center as accurately as one can, I suppose, given the differences in paper and conditions, etc.
However, there was a guy who used to post pictures of groups shot with his .223 where there was clearly un-burnt paper between the holes and he would mark them as groups waaay below caliber - some in the mid to low teens, as I recall. When challenged, this character (like some do) took it as a personal attack and get really ignorant - stoopid, in fact. I saw him brag about some of the same groups on the benchrest.com board and get flamed to a cinder for it (and deservidly so!). It made for some humorous reading for sure!
Having said that (coma!) someone made mention of the paper between holes as evidence the groups couldn't be as small as claimed. There was a time when I might have agreed. However, in order to get a better view of the actual burned outside hole edges, I push the curled (burnt) edges of the hole back to the hole center and put some tape on the back of the group to hold the paper in place. Then I can clearly see the hole edges to take a measurement.
But, this process does sometimes push paper fragments between some holes. The burned edges don't change position, and that is what is being measured. However, the shards of paper that appear the center of the group do give rise to someone not knowing what has happened to question the measurement results, especially if there is paper showing between the holes and results are sub-caliber. The shards present a distorted result - not the truth - but do give rise to questions, sometimes. (The group on the lower right shows what I mean. After ironing the edges out, it looks like one can see several "rings" which might be misconstrued as circles depicting the actual holes, which may or may not be true.)
Also, as someone mentioned, using card stock or heavy paper for targets results in much cleaner holes and aids measurement consistancy a lot, I find too.
I agree w/ you: Center to center via the distance between the burnt edges of the group, minus the average distance between the burnt edges of one or more discrete holes is what I use for my own records and what I'm referring to when posted here. I realize it ain't perfect, especially when taken to .000. However, for the sake of discussion which is usually load development results, they are fine, and I won't question anyone elses measurements, even if I'm dubious
