sucesss!! well, sorta

[Jack]

Feathermax, I measure groups the way you do- and the way Billy does.
I do a horizontal measurement, edge to edge, of the bullet holes at the widest point. Then the same thing vertical. Then take the larger number and subtract one bullet diameter to get the center to center measurement, as Billy described.
1.25 inch groups out of a rifle are all you need to kill any big game animal at any distance one would prudently shoot.
Does a 100 yard group indicate good performance at 200 and 300, etc? Usually.
If you shoot a 1.25 incher at 100, it's likely going to be a 2.5 incher at 200, although there are exceptions. Some rifles even do better at longer ranges.