How to measure group size

by B.B. Pelletier

I’ve covered this material before, but it was long ago, and we’ve grown in readership lately. New shooters read about group sizes and assume there’s an accurate and foolproof way of measuring them. That’s simply not the case. The great barrel maker Harry Pope once spent half an hour with a magnifying glass and a caliper to measure the size of a group; and in the end, he revised its published size by about 1/64 of an inch! So, if a world-class marksman (he held several world records) and barrel maker has trouble doing it, what hope is there for the rest of us? I’ll address that question at the end of this report.

I’m not going to delve into the history of measuring groups, because there’s too much material to cover in less than a book chapter of perhaps 5,000 words. And most of you would either be asleep or reading another website if I got down to the minute details involved in the dozens of different ways it has been done over the ages. So, today I’ll plunge right into the subject with the belief that we all measure our groups the same way and don’t care how it was done 100 years ago.

It’s customary to measure shot groups from the two centers of the bullets that are farthest apart. But the new shooter wants to know how to find the center of a hole reliably. The answer is simple — don’t look for the center, find the edge. If you measure from the extreme edges of the two bullet (or pellet) holes farthest apart and subtract one bullet diameter from the answer, you have just subtracted the radius from each of the bullet holes. The radius is the distance from the center of a hole to its edge — so in fact you have just “found” the two centers of the holes in question.

But where’s the edge?
To do it this way, you need to find the edge of each hole, and that’s where it gets hard. If you’re shooting wadcutter pellets, the holes will be sharp, round and easy to measure; but if you’re shooting domed or pointed pellets, the holes will be much more vague. This is where those who shoot centerfire rifles have it easy compared to airgunners. Because their bullets go so fast, they leave sharp, round holes in the target regardless of the shape of the bullet.


It’s pretty obvious why wadcutters are the darlings of the paper target crowd!


This is target paper, and yet these Beeman Kodiak domes leave a vague hole. Measure that!


Even pointed bullets tear clean holes when they rip through targets at close to 3,000 f.p.s., like the 87-grain spitzers from this .250/3000 Savage.

Where the edge of the hole really is…is always a guess. Always! But with careful work, you can get the error range down to just a few tens of thousandths of an inch.

The equipment
I use dial calipers for measuring group sizes. I don’t own a digital caliper, so I’m relying on a caliper that has a mechanical dial calibrated in thousandths of an inch. Actually, the dial is just part of the measurement. For the gross measurement, you have to read the scale on the staff of the caliper, as well. The dial just gives you the final thousandths of an inch.


A dial caliper is an easy way to measure groups, but a ruler will do if that’s all you have.

When I measure groups, I do not look at the dial. I adjust the jaws until they just touch the edges of the two holes I’m measuring. This usually takes several attempts; and the stronger the light, the easier it is to be precise. The target paper must be flat to do this right, so the best way is to lay the target on a well-lit table and then place the dial caliper over the group and work the caliper jaws down until they touch just the edges of the holes. If there isn’t enough light on the target, I’ll use a powerful flashlight — a tactical flashlight — to illuminate the target.

Can groups be measured without a caliper?
Yes, they can. In that case, you need the most accurate ruler you can get your hands on. A steel machinist’s rule is the best tool to use. But lacking that, you can measure groups with an ordinary school-grade ruler. However, once you’ve taken the measurement, it’s best to convert to decimal fractions that run out to thousandths of an inch — because nobody will be impressed with a group that measures 3/8 inch, but everyone will admire a group that’s 0.375 inches! I’m joking, of course, but not as much as you would like me to be.

Finding the edge
Have you noticed that my groups often end up in the white target paper instead of in the black bullseye? My brother-in-law thinks that means the guns I shoot are inaccurate because they don’t hit the black bull — but I do it so I can find the edge of the pellet holes easier.

When you look for the edges of the holes, there are clues. Sometimes the pellets will leave a round lead ring where they tear through the paper. You can see this on the white target paper but not on the black. Other times there will be rips that extend radially away from the edge of the hole. These rips are part of the diameter of the pellet or bullet, as I will now show you.


See how the bullet hole in the white stands out but in the black is obscure? Much easier to measure this .45/70 hole when it’s in the white. Notice, also, that the radial rips are part of the bullet’s diameter.

One way to get the best results is to push a pellet into one of the holes (try not to use either of the two measuring holes for this) and see how the skirt of the pellet passes through the hole. That will give you a good sense of where to put your caliper jaws in relation to the rips or the lead ring.

Accuracy
I’ve said this before, but it bears repeating: any group measured to the thousandth of an inch is off by up to twenty-five thousandth of an inch. I may have even said a larger error number than that.

Why are people so “inaccurate”? Well, do you know what twenty-five thousandths of an inch looks like? For your entertainment I will now illustrate how large that is.


Twenty-five thousandths of an inch sounds big, but when you see it illustrated here as the space between the caliper jaws, it really isn’t. If you can measure a group this close to actual size you’re doing very good! The silver dime is on sabbatical, so this one is a stand-in.

When doctors disagree…
…who shall decide? If you could only attend one regional target match and watch competing coaches argue with the officials over measurements of a few thousandths of an inch, you would soon understand that measuring targets is an imperfect science. Always has been and probably always will be. At the Olympic and World Cup levels, the error has been reduced to less than a thousandth of an inch with sound transducers and triangulation…but it still isn’t exact.

What hope do we have?
If this all sounds like magic or hokum, consider this — the more often you do this the more uniform your measurements will become. You may never get the measurement exactly right, and there’s virtually NO way to prove that you did or didn’t, but you will tend to err in the same way time after time. In time, your measurements take on a standard error that’s based on YOU. You become the standard!