How To Measure Group Sizes
Well, Zak, looks like you've put your finger on an area of disagreement amongst the various target shooting Associations. Over at ASSA, the plan was to try to match up with the type of judging done in Bullseye competition, where the scoring is based upon shots being inside one or another of the numbered rings, smallest being best score, etc.
The NBRSA method simply measures the center distance between the two shots in a group that were farthest apart, a one-axis Caliper measurement that adjusts for the size of the bullet. The advantage is that the measurement is so simple that it can be done in the field without any complicated calculations required. It also gives an artificially good result compared to running a circle gauge.
Depending upon what you are used to doing, either approach is fine, as long as everyone understands that you are comparing apples and oranges.
Here is why the two methods can give very different scores:
Just to make everybody happy, ASSA has decided to include results calculated both ways, so everyone can know where they stand, no matter whose method is used. We think the NBRSA method gives "Rosy Scenario" results in some situations, per the example we've posted, but they've been at it a while, and we'll just make another score column to try to be all-inclusive, "big tent" kinda guys.