Measuring the sizes of groups
This report covers:
- Data mining
- How to measure group size
- Where’s the edge?
- The big question
- It’s always a guess
- The equipment
- Can groups be measured without a caliper?
- A new tool
- Finding the edge
- The bottom line
- When doctors disagree…
What hope do we have?
I’m going to discuss measuring group sizes today — how measurements are obtained and the accuracy we can expect — of the group size measurements, not the guns that shot the groups.
I have written quite a bit on this subject in the past. I’m not going to rewrite all of that today. Instead I copied what I’ve written and the I edited it to reflect the lessons I have learned since those reports were written. So, yes, this report was easy to write, but it still took a long time because I had to edit as I went.
How to measure group size
The method I’m about to explain is the same one that was used by Harry Pope at the turn of the 20th century. It is simple, fast and easy to do. It’s also open to interpretation and small errors. Are the hairs standing up on the back of your neck, yet?
It’s customary to measure shot groups from the two centers of the bullet holes 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/pellet diameter from the answer, you have just subtracted the radius from each of the two outlying 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.
You bracket the group on its largest dimension with a dial caliper, so that one jaw touches the extreme edge of one hole and the other jaw touches the extreme opposite edge of the hole farthest away from the first hole. It sounds easy and would be except for one thing.
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!
These Beeman Kodiak domed pellets leave vague holes. Measure that group!
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.
BB often “cheats” by using small scissors to cut away the torn paper around pellet holes, so they show up better in a picture. He has to be careful to cut those paper pieces circular so they look like the pellet passed through.
The big question
Here’s the question many of you have asked in the comments to this blog, and many more have wondered privately — how is it possible to identify the exact edge of a pellet hole with a caliper or any other measuring instrument? It’s not.
It is not possible to locate the exact edge of a pellet hole with a measuring instrument — whether it be a dial caliper or the index marks on a ruler. The hole is insubstantial, and you’re trying to measure it as though it was solid. It can’t be done — not with great precision, anyway.
It’s always a guess
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 thousandths of an inch.
When I measure groups, I do not look at the caliper dial or the digital readout. 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?
It is possible to measure groups with a ruler, but a caliper does make it easier.
A new tool
Just before Christmas last December, reader David Enoch contacted me about a new tool called the Group Mate that was developed in the United Kingdom. David bought a bunch of them and sent one to me as a gift. Here is what I have found thus far.
The Group Mate tool from Snappy Targets is for measuring group sizes.
I find this tool works best on light-colored paper because the lines disappear when they are over black bulls. Also the circles are for encompassing the entire group — not for measuring the center-to-center distance. In the inch-pattern circles at the bottom of the tool, the hash marks are graduated in one-eighth inches.
I haven’t used the tool that much yet, but if I wasn’t always shooting at bullseye targets, this might be the way to go.
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, Bob, used to tell me that he felt the guns I shot were inaccurate because they didn’t hit the black bull. However I sometimes shoot groups in the white so I can find the edge of the pellet holes easier. Bob now shoots groups of his own with firearms and pellet guns every week and he understands what I have been saying.
When you look for the edges of the holes, there are sometimes 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.
The .45/70 lead bullet leaves a distinct dark ring at its edge when it punches through paper — even at just 1,200 f.p.s.! But that ring can’t be seen in the black bullseye.
I’ve said this many times 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’s measurement of groups 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.
People talk about 25 thousandths of an inch. This is how large it really is.
The bottom line
I resolved not to obsess over this issue years ago. Because if I couldn’t get past it, I couldn’t write about airguns. In the same way that I know that chronograph readings are also not exact, I know that the closest I can come to an exact measurement on paper is probably 0.005-inches, when everything goes my way. But give me ragged BB holes to measure and a paper target that rips instead of showing clean holes, and the error is probably closer to 0.020 inches. And that’s on a good day, when I am really trying my hardest. But the number I publish will always have two or three decimal places, and it will look official to everyone.
Yes there are tricks like putting tape on the backside of the target to limit the paper from tearing, using stiff cardboard the same way, and sometimes these techniques do work well, but other times they don’t. I use cardboard all the time.
When doctors disagree…
…who shall decide how large the group is? Or where its edges touch? 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 have become the standard!
Measuring group size is something almost every shooter does. Most of you know how to do it, but we have many new readers who perhaps have never been taught. This report was for them.
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