My problem with measuring from C 2 C of the widest shots is this:

Say I shoot a 3shot group and they're all three equidistant from eachother, i.e. an equilateral triangle, all three sides and angles the same...measuring from one side to the other, say all sides are 3" C 2 C. That's a 3" group if I measure it C 2 C, but in actuality, it's a 3.46" group...each shot is 1.73" from the center of the group, so if you shot 100shots that fit under the same circle as those three, you'd get a circle that was 3.46" in diameter, not a 3" circle.

Basically, where your bullet can strike is similar to a shotgun group, it's a conical path, so if your rifle is accurate to 1MOA, if you locked it in and shot hundreds of rounds with NO fliers, you'd get a circle at 1MOA, where 2 or three given points can be closer than that and still be ON THE CIRCLE, not just inside of it.

Measuring the two worst shots C 2 C assumes that one is exactly in the center and the other is the farthest from the center, so the other one is a "better shot" and can be disregarded. That's why the "center mass" method is what I use. It also helps you pin down how far EXACTLY you have to adjust your scope to center your group around the target.

Really to get more exact than the center mass 3 shot method, you've got to shoot more rounds, and even then the center mass method will give you a better representation of your accuracy.

Of course, most guys can't shoot consistently enough in their own form to make EVERY group the same, even in a vise, so the C 2 C method works, but if you only measure between the two holes farthest apart, why do you shoot three? or Five? Or ten?