How is a group size determined?

[huntsman]

Here's one for you target fellas:

How exactly does one determine the size of a group of shots on a target? All my life I've heard of such-and-such inch groups, and I always thought that meant the distance between the outside edges of the two most widely spaced bullet holes. But now I am reading that folks shooting .308 caliber are getting 3/8" (.375 inch) groups, which would mean nearly one-hole groups.

I'm also finding that there are significant differences between the average distance between points of impact on separate groups with the same outside diameter measurement. This factors into accuracy to some degree.

So, what is the official means of measuring the group size?

[TennesseeNuc]

huntsman,
I believe that group size in most commonly measured from the center of the holes.  With that being the case a 3/8" group would be one ragged hole.
Best,
TnNuc

[ajj]

Groups are measured  "center to center." (If you fired one shot you wouldn't think you had a .308 "group" and if you had two rifles of exactly equal accuracy you wouldn't say that the .223 was more accurate than the .308 because the total size of the hole was smaller.) Some people try to measure outside to outside and then subtract one bullet diameter but without a dedicated measuring tool you usually end up with an optimistic group size because the hole in the paper swells shut slightly. For plain old "let's get pretty close" measurements I just put the points of a dial caliper in the centers of the two most widely separated holes. Your eye can find those centers with remarkable accuracy and I think that yields better measuremants than trying to judge where the real edge of the hole starts. JMHO.

[ajj]

I should add that for benchrest competition and for true, one-hole groups, the above method doesn't work so well. I don't have that "problem."

[ajj]

I should add that for benchrest competition and for true, one-hole groups, the above method doesn't work so well. I don't have that "problem."

[Dwarfmiester]

There are several ways to do it, the one we use is a attachment that you use on a dial caliper.  It has a number of concentric cirles on a clear plastic plate, place the hole of the first bullet in the center of the correct size circle and hold it in place while you move the adjustable side till the furthest hole is centered.  Then just read off the measurement from the dial.  The best part of this is with a little practice several different people can get the same result, usually within .001 to .003.  You may have to measure a group several different ways then you would take the largest.