How shot groups are measured
by B.B. Pelletier
Believe me — there’s enough information on this topic to fill many reports. I will do that if there’s enough interest; but if interest is confined to just one or two people, I’ll recommend that you read several of the gun books that I listed in my Building an airgun library blog.
Those books present and discuss several ways of target measurement that are considered outdated today, but which hobbyists keep trying to reinvent. One is the old string measurement in which a piece of string is stretched between the center of the target and the center of each bullet (pellet) hole. The cumulative length of the string then determines the cumulative distance of all the shots from the center point of the target. This system of measurement was popular in the late 19th century, having replaced a simpler method in which the string was stretched around pegs placed in all the bullet holes and gave the “circumference” of the group.
That small paragraph is all I’m going to say about these older group measurement methods unless I see a reason for more. Today, I want to concentrate on how groups are measured and reported these days.
Here we go!
Are you just a little bit anal? Don’t answer that. Because you’re an airgunner, we can tell there’s a missing chromosome in your DNA that drives you to examine minutiae and project worlds onto what you see. Please don’t be insulted, because look who’s talking — Mr. “The atomic clock in Denver may be accurate, but everyone knows that it’s off by just a little!”
I feel the tightness of your headband when you’re confronted by numbers. Know what I do about it? Like every other gun writer — I lie (sort of…although most people wouldn’t call it lying). I give you numbers out to three decimal places, knowing that you will focus on them as though they have been transcribed from court records. What I seldom do (other than right now) is admit how far off those numbers might be. So, today, is honesty day and I’m going to tell you exactly how I measure targets.
Oh, and by the way — there’s only one other way of measuring targets that is any more accurate than the one I will show you, and that is sound measurement. At world cup and Olympic matches, the targets are scored by sound transducers that triangulate the sound of the pellet tearing through the target paper to extreme precision. But at U.S. National Junior Airgun Matches I’ve attended, they guess at the location of the pellet holes just like I do. Yes, I said guess, and anyone who disagrees with me will be sent outside to meet with my friend Mac!
There! Have I upset everyone? If not, please leave a comment, and I’ll insult your children, spouse and pets.
Here’s the deal. Determining the location of pellet holes today is a bit like invoking the Heisenberg principle, in which that which we observe is also altered. The most formal way of doing it today (other than the sound measurement mentioned above) is by sticking a plug called a scoring gauge through the pellet hole and looking through the magnifying edge of the plug to see what is the highest-scoring ring touched by the pellet. In international competition, the line must be broken by the pellet. It’s a subtle but important difference.
For a great article on scoring gauges, read Gary Anderson’s article located here. Gary is the Director Emeritus of the Civilian Marksmanship Program (formerly the Office of the Director of Civilian Marksmanship) in the U.S., and he’s also a double gold medal Olympic high-power rifle champion. He has had his targets scored more than once and is most familiar with the problems of the sport. It was through him at the now-defunct Winston-Salem Airgun Exposition that Edith and I were first exposed to sound-scored targets.
These gauges or plugs can and almost always do enlarge the holes left by the pellets, so it would be possible for an unscrupulous person, like a team coach, to “scooch” the plug in the direction that best supports his team when he inserts it in the hole. Or, if you make the coaches of the opposing teams score each other’s targets, the elongated holes will run the other way. Don’t think it doesn’t happen — I have seen opposing coaches almost come to blows over how the targets are scored. At the national level, they don’t allow coaches anywhere near the scoring until the deed is done. Then, they get to examine their team’s targets and argue for any close calls they find. And they DO argue!
Another way to score a target is the optical method, in which a device is used to locate the pellet hole without damaging it. I have owned and used an Eagle Eye device for the past 15 years, and it works quite well — except for one thing. You are still GUESSING where the pellet hole is when you do it this way. It works good enough for regional-level matches where the targets have scoring rings set at prescribed distances, but only for calculating the score — not for measuring the size of shot groups — which brings us back to today’s topic.
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?
You measure group size by bracketing the group 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.
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 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.
But once you do your best to find the closest measurement across the two widest holes, you’re left with a number that has two or three decimal places. It sounds or reads like you have great precision, when in fact the best you could do was make a guess where the boundaries of the holes were. Harry Pope struggled with the same thing a century ago, and there is nothing anyone can do about it. Pope wore two pairs of glasses and also used a magnifying glass to measure his targets, and he still was only guessing at where the shot boundaries were. He took as long as 30 minutes to carefully examine important targets this way. I seldom take longer than a minute, and frequently a lot less than that.
So, all of us gun writers continue to bracket our groups with dial calipers and make a best educated guess where the edges of the two outlying holes are, then we subtract one pellet diameter and give you the number. We subtract one pellet diameter because what we really want is to measure the distance between the centers of those two pellet holes. Subtracting one pellet diameter from the overall reading takes half the diameter away from each of the two holes we used to bound the group. Thus, we get to the centers.
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 guns. 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.
Here’s an exercise that will illustrate the dynamic I’m explaining. Which sounds more precise — 3/4-inch or 0.750 inches? If you’re honest with yourself, you know the decimal fraction sounds more exact. The point is that both of them are being obtained from a system that has built-in tolerances for slop!
Think you can measure this group to the nearest thousandth? Bully for you, because this is as easy as it ever gets! You will always be off by as little as 0.005 inches and as much as 0.020 inches when the holes are this clean. These are holes left by wadcutter target pellets.
Now where are the holes? This is what domed pellets look like close up. Where are the edges?
I need a vacation! This is what slow-moving BBs do to a target that’s been attached to a cardboard backer. Guess where the holes are?
I’m sure many of you knew this already and didn’t need to be reminded. But from some of the comments I’ve been seeing recently, I was concerned that some of us are getting hung up on the numbers — as in accepting them at face value. These numbers are a best guess and are published with the best of intentions, but they are, and always will be, a little off.
Here’s what you can say about such numbers. A 0.36-inch group is unquestionably tighter than a 0.511-inch group. Even when the first group is shot with .25-caliber pellets and the second is shot with .177-caliber pellets, so the two groups appear very much the same, the first one is still tighter.
What I’m saying is that these numbers can be used as relative measurements. Just don’t stake your career on them. This is one more good reason why I shoot 10-shot groups when possible. Not because the measurements are any more precise, but because there are always more opportunities for the gun to mess up. If all 10 holes are in close proximity — even if my estimate of how close is wrong — you still have a good idea of how well the gun is shooting.
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