John, I understand your reasoning.
My point is, the accuracy of a rifle and its ammo is the largest group they shoot. Half its size is the furthest a bullet will strike from where the rifle's aimed. If the rifle's aimed somewhere inside a 1 MOA circle 'cause that's the area it moves around in when held by a shooter, the furthest a shot will land from the edge of that circle is half the size of the rifle and ammo's accuracy.
So, a shooter aiming at a point on the target holds within 1/2 MOA of that point; his area where the sights align is 1 MOA. His desired impact point's in the middle of it. His rifle and ammo's largest groups are 2 MOA. The furthest a bullet will strike from where it's aimed is half that amount; 1 MOA.
Therefore, the resultant group on target should the shooter fire many shots will be 1 MOA of holding plus 2 MOA of accuracy and that's 3 MOA. The shooter's shot will land somewhere between his desired impact point and 1-1/2 MOA away from it. The fewest number of shots will be at the outside edge, but they're still gonna be there. They have to be counted in the measurement of the group he shoots. Most of the shots will be inside about 2 MOA, but not all of them. About one third will be in the 2 and 3 MOA range.
In talking with ballistic folks at Lake City Army Ammo Plant on their 7.62 NATO match ammo tests, I've gained some insight on the realities of accuracy tests. They would shoot a couple hundred shots per test group and they told me the following. It's been their observations for a "rule of thumb" that about 10% of the shots fall in the outer 10% of the group radius, 20% in the next inner 20%, 30% in the next on and finally 40% in the inner 40% of the group's radius. Which correllates well with the mean radius (their standard measurement of accuracy) covering about 70% of the bullet holes; inner 40% and 30% of the shot holes and about 70% of the group's radius. Oft times there would be 5 or 10 holes by themselves at the outer edge of a 200-shot cluster typically about 6 to 11 inches in diameter depending on the quality of the ammo lot tested. Yet those outliers very precicely represented the furthest a bullet would strike from group center or where the rifle was aimed and have to be counted as part of the group. I've seen a couple of their test targets and it's hard, but one can fairly easy identify each bullet hole for measuring its location with reasonable accuracy. They're fired at 600 yards.
Where the rifle's aimed about a desired point for bullet placement by the shooter, any error in that has to be added to the radius of the shot group to determine the maximum miss distance from the desired point of impact.
Last edited by Bart B.; October 12, 2012 at 08:15 AM.