From the October 1952 issue of American Rifleman, a question from a reader on how to properly measure shot groups.
What is the proper procedure for measuring five- and 10-shot groups fired in accuracy tests of ammunition and rifles, and in benchrest matches?―Lt. Col. Louis E. Roninger, Oak Ridge, TN.
Answer by L.R. Wallack: Groups are measured from center to center of the two outermost shots. This may be done by whatever means available, and it’s my belief that a man who can properly read a scale can come as close as any but the most precise optical measuring device.
A fast and efficient method with the scale is to measure the total group, outside to outside, then deduct the diameter of one bullet. It is a lot more accurate than trying to center the hole in a tight group.
Record groups sent in to the National Bench Rest Shooters Association are measured on a Bausch & Lomb comparator to 20-power magnification. The target is then measured on a grid for total displacement of paper in the group, the diameter of one bullet is then subtracted. Or, more properly, the displaced paper for one bullet hole is deducted. These readings are very close.