This might sound stupid. I have been shooting a very long time and I am really not sure the proper way to measure a group. I am just not quite sure how to do it. Anybody out there that can give me the proper way? It would be greatly appreciated. I have some groups to measure that are really tight.
I do the same thing Ed does, just with a transparent ruler. Sometimes it's hard to tell which two are furthest apart just by eyeballing them, so I measure all the combinations that look like they are close to being furthest apart.
"Chances are, when we meet intelligent life forms in outer space, they're going to be descended from predators." - Michio Kaku
"...really not sure the proper way to measure a group."
Do it any way you wish but the common definition of group size depends on where the centers of the two widest holes are, that avoids any measurement difference between bullets of different diameters. Ed tells how it's done in ocmpetition.
The legal measurement for matches is a good standard for matches. My opinion is that hunting firearms are better evaluated for hunting with the measurement of ATC, or average of shot centers to the center of the group. This gives a clear picture of how far shots may vary from the point of aim for a firearm that is sighted in at a particular distance.