Groups
Measure from the outside edges of the two that are farthest apart and subtract a bullet diameter.
This only gives credit to the weapon for its worst two shots, and none to
any that were closer. Useful in telling you that with a given zero, that the farthest from the point of aim that any shot will strike is half the group size...(2-inch group...the greatest distance from point of aim will be one inch.) This is assuming that the shot at the target is taken under the same conditions as the test group.
A better test of accuracy is to determine the center of the group, measure the distance of each shot from that point...and calculate the average.
Precise triangulation of the group center is a little complex, but the
"Field Expedient" method is nearly as good, and much easier. It's not
as precise, but it will do.
Using a straightedge, draw a square with its lines on the tangent of the
outer bullet holes. Keep the square as close to having 90 degree sides as possible. When the square is established, use the straightedge to form an "X" by drawing two intersecting lines from corner to corner. The center of the X will be the center of the group. It may not agree with the point of aim.
From that center, use the straightedge to determine the angle of departure
from the centerpoint to the center of the bullet hole... measure the distance from the center of the X to the outer edge of the hole on that angle, and subtract one-half a bullet diameter.
Do that for each hole, and find the average distance. That figure will
determine the average deflection from point of aim that you can expect with any given shot. Doubling the figure will determine the "true" group
size...or the worst that you can expect from the rifle on a given shot.
The "true" group size is a better indicator because it gives credit for all
shots, rather than only the two worst ones.
To test your own accuracy with the rifle, repeat the test by firing a 7-shot group from a field position...Your choice of standing, sitting, or prone...or
all three if you have the time and the ammo to burn. This will tell you what
the rifle/ammo combination is capable of in your hands under realistic
conditions. Be prepared for an humbling result. If your field group is three times the size of your benchrest group, you're an exceptionally good shot, and well above average. If it's only twice the size, you are sniper class. If the two are the same, you're one in about 10,000...a Master Class marksman.
Luck!
Tuner