Shot group sizes. How are they "officially" measured?

[230RN]

I found this remark on an informal review of ammuniton:

Groups were measured outside edge to outside edge... the real way. There was only a very slight breeze blowing left-to-right. Temp was about 75 degrees F and lighting was slightly overcast if these sort of things matter to you. I'd say shooting conditions were excellent.

In reference to "outside edge to outside edge... the real way," I always figured that groups were measured from center to center of the widest shots. However, I had been out of shooting for a while and a lot has changed.

I realize that "outside edge to outside edge" will give you an indication of whether or not a bullet is likely to make contact with the target. I recall that one reason .45s were preferred over .38s in "stand up on your hind legs with your off hand in your back pocket and hold the gun out with one hand" target shooting was because a shot placed in the exact same spot with a .45 as with a .38 is more likely to touch a scoring ring because the diameter of the bullet is larger.

I also realize that it is easier to measure the groups from outside edge to outside edge, but I don't see that taking measurements center-to-center of the widest shots is that much of a problem.

But for comparing accuracies, it seems that center-to-center measurements are more appropriate. I always assumed that in The American Rifleman accuracy testing, they measure groups center to center, and as far as I know, benchrest shooters measure them center to center.

So what did the reviewer mean by "outside edge to outside edge... the real way?"

Ref:

http://www.thehighroad.org/archive/index.php/t-96062.html

 

[TexasRifleman]

I always figured that groups were measured from center to center of the widest shots. However, I had been out of shooting for a while and a lot has changed.

Thats the way I've seen it and at several competitions I've seen judges use a little plastic overlay with circles the size of different calibers that you hold up to the target, all with a dot in the middle, and place a ruler on a couple of these and measure dot to dot, getting center to center.

I guess it depends on whose "official" rules you use.

 

[Jim Watson]

Competitive benchrest shooting figures groups center-to-center.

If your gun's accuracy or the range being shot at gives you individual bullet holes, you can get that number by measuring from the inside edge of one wide hole to the outside edge of the other. This gives you actual lines to measure from instead of trying to center over a hole in paper.

If you were figuring the bullseye scoring possibility of gun, assuming it perfectly zeroed, you might measure to the INSIDE edges of the holes.

I see no point in measuring to the outside edge of the holes except for convenience. A lot of people say "subtract one bullet diameter" but that is imprecise in a paper target.

"The real way" is sense free in this quote.

 

[csmkersh]

Outside edge to outside edge minus bullet diameter. This, in effect, gives you a good center to center measurement.


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[The Lone Haranguer]

AFAIK, center to center of the widest-dispersed shots. It is easier to measure from the inside of one hole to the outside of the other with a rule or caliper than to "eyeball" the centers of the holes. We must also consider what quality or property we are trying to quantify with group sizes - the accuracy of the firearm or the accuracy of the shooter.

 

[williamthedog]

csmkersh;
Outside edge to outside edge minus bullet diameter. This, in effect, gives you a good center to center measurement.






that is how i do it.
except i take an example shot and measure that.
and then minus the diameter of that hole.

 

[230RN]

OK, thanks all.

I knew you could take ODs and subtract, or IDs and add, or ID at one hole and OD at the other. I also have used the little clear plastic scoring jigs. But I wondered about whether there was a new way to report the group size that came about while I was sleeping.

I guess maybe that's what that reviewer meant, though --that that was the way he measured the dispersion, by ODs and subtraction, per williamthedog's post.

TNX!

Terry, 230RN