How are shot groups measured?

Aaron

To get back to your original question, Benchrest groups are and always have been measured as the distance between the two farthest apart shots.

There are several ways to measure groups but the current method is the best, considering the size of the groups being fired. It is quick, easy for scorers to learn, and fair. Preserving the historical record is probably the best reason for not considering any other.

Long range (600 & 1000 yard) groups, OTOH, could be and probably should be measured differently. The greater possibility of shots off the paper, and the added penalty for such, is reason enough in itself. But, preserving the records will probably override any changes, ever.

JMHO

Ray
 
Xfactor Do you actually compete/participate in BR? Just curious. several times a year we get someone who has never fired a shot in a match but knows that we are doing it all wrong and is here to save us from ourselves. I'm not picking a fight just curious.
 
The flaw I can see in trying to measure a group with an inscribed circle is that it requires an assumption. That being that the hole in the target is bullet diameter. It rarely,if ever, is. Additionally, if the circle is to be inscribed on Lexan or some other transparent material the width of the inscription would enter into the accuracy. To be able to cover all possible group sizes would require a very large number of templates. The accuracy of the measurement would be further impaired by increments of diameter of the circles. In reality, measuring with a template could be reduced to .005/.010 increments. The current method seems to work well. It involves a template attached to a digital or dial caliper upon which is attached an inscribed Lexan plate. The proper diameter circle is visually centered over a bullet hole. The caliper is then opened to the widest bullet hole and again recentered over that hole. The reading on the caliper is the center to center distance between the widest shots. If there are similar hole dispersions this may require multiple measurements; the widest being the group size.

Mike Swartz
 
the problem lies in your opening statement.
"I'm wondering if the group size is measured simply as the distance between the two farthest apart shots, or if it's calculated very precisely using the diameter of the smallest enclosing circle..."

niether method is used( forget that the statment has a very baised wording implying that YOUR proposed method is "very precise" implying what we do is not precise,,,,get a life)

you have been told by no less than three people how br groups are measured.

neither of the two methods you describe are used.

your insistance on the "
center point" is all wrong, and you will not let go of it.

it is not smantics. it is the difference of how benchrest shooter measure groups in the real world and a person with a preconcived idea trying to write a program that has no real world application in the benchrest world.

close your mouth, fingers off the keys, open ears, read the words that are written( not what you THINK they say) .

look up two words: calculation, and measurement

three times you have been told, you still don't get it.
we know what we do and how we do it. IF you had any real world experience in this field you would understand.

mike in co
 
Cheechako said:
...To get back to your original question, Benchrest groups are and always have been measured as the distance between the two farthest apart shots.
Long range (600 & 1000 yard) groups, OTOH, could be and probably should be measured differently. The greater possibility of shots off the paper, and the added penalty for such, is reason enough in itself. But, preserving the records will probably override any changes, ever.
Click to expand...
Thanks Ray. I'm not trying to invent an "new-and-improved" system or anything... I just wanted to know how it was done in BR competition so I could use the same method. (And then somehow we all got sidetracked into a debate about how the exact measuring point of each shot is determined... )
Anyway, thanks again to everyone for all the helpful input.


Jerry Dailey said:
Take a look at the device swown at www.neiljones.com/html/target_measuring.html Something similar is used at all the matches I shoot at.
Click to expand...

That's cool, Jerry - thanks for the link.
 
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Xfactor;
Check out a BR match. Measuring groups is a lot easier to imagine when you see it first hand. The people that do the job have usually been at it for sometime and there are very few problems. IF you go to a score match then the position of the bullet relative to the "X" ring is all important. In group shooting the group can form anywhere within the record box. Relation of the group to the aiming bull is shooter preference and is not a scoring issue.

Good Shooting;

Mike Swartz
 
Xfactor,


I've just got to jump in here and offer you some small support ;) Kudo's to you for bearing up under the brunt of a full frontal assault! Classy. Your use of "smallest enclosing circle" threw some of us for a loop. You're exactly right except that in match-measuring a line is established through the long axis of the group and the measurement is made directly. Your description is apt, in fact once the theoretical center has been plotted then "smallest enclosing circle" is probably the clearest description I've heard..... BUT!! :D ....... not too awful useful in the real world.

(Mike, you're 'way out of line man.... the guy DOES get it! I feel that you're doing some damage by your insistence, LIGHTEN UP MY FRIEND! :) )



al
 
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I don't want to get in on the heat of the discussion but just wanted to add for those who care, the method I worked out.

While in very small groups as done in this forum (short range BR) my method doesn't work all that well but could be adapted pretty easily. I also have an idea for an electronic measuring system but I tend to shy away from that sort of thing cause it's always too easy to screw up. No matter how it's done, there's always some subjectivity to it because we need to use our eyes.

I purchased a 12" Dial Caliper and an Optical Center Punch Optic. Using the center punch lens, it is incredibly easy to center on a bullet hole or on the meplat hole when dealing with 1K targets. 1K targets don't have a clean hole, only a meplat hole. So they're easier if that is the way you measure. In PA we do that but in the IBS, they use the grease spot formed by the bullet. It could be said that is more consistent when you consider that in 600 yard there's some holes that are clean and others that are not, depending on velocity.

Either way, the optic I use is in a sleeve which is welded to a machined area in the jaw of the caliper. By using the same optic for both holes, you end up with a measurement that has as little mechanical error as would be in the instrument itself. Other error is a maximum of twice your optical centering error on the holes, which I feel is pretty slight. I've measured a target 5 times without seeing .002 of variation in the measurement. I consider that to be about as close as they can be done. When 3 or 4 people come within 2 thou of each other 3 or 4 times each, I feel good about the average number.

Various graduated loupes or other optics can be used for this as well but the graduations on them are not in any mechanical center of the instrument. So, they have built in error that the optical center punch does not. If the orientation of that loupe isn't the same each time there's additional error that could be really significant.

fwiw, the optic for the punch is roughly $40 at McMaster.
 
alinwa said:
I've just got to jump in here and offer you some small support ;) Kudo's to you for bearing up under the brunt of a full frontal assault! Classy. Your use of "smallest enclosing circle" threw some of us for a loop. You're exactly right except that in match-measuring a line is established through the long axis of the group and the measurement is made directly. Your description is apt, in fact once the theoretical center has been plotted then "smallest enclosing circle" is probably the clearest description I've heard..... BUT!! :D ....... not too awful useful in the real world.

(Mike, you're 'way out of line man.... the guy DOES get it! I feel that you're doing some damage by your insistence, LIGHTEN UP MY FRIEND! :) )
Click to expand...

Thanks Al - I do appreciate the support!
The smallest enclosing circle thing is definitely interesting, but I'm actually glad that it's not how groups are measured in BR matches - which in my mind is the pinnacle of the accuracy shooting sports - b/c it's really complex to calculate (even after the challenge of finding the theoretical center point of the bullet holes).
 
Hi Guys,
The ASSRA measures there Benchrest for score matches from the center of the bullet hole, I cant imagine how they do something like that with any kind of accuracy.
Not sure how they measure there group matches.
Set-Trigger
 
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