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Xfactor
09-09-2008, 11:44 AM
Can anyone confirm how shot-groups are measured in benchrest Group competitions? 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...

Thanks,

- Aaron

HovisKM
09-09-2008, 12:11 PM
Center to Center of the two widest shots.

Hovis

Xfactor
09-09-2008, 04:21 PM
Thanks Hovis.
It's interesting that group sizes are calculated simply by taking the distance between the 2 farthest apart points (center to center)... if 3 shots form an equilateral triangle, the smallest enclosing circle for these 3 shots will be significantly larger than the distance between any 2 of the 3 shots. (close to 20% in my rough calculations... )
Although I guess I'm not surprised, as the fact is that calculating the smallest enclosing circle is actually a known computational geometry challenge... there's really no simple way to calculate it. I've been working on a script to calculate it for me, but if it's true that this is not how benchrest groups are measured at all within the sport, then what's the point?! ;)

Mike Swartz
09-09-2008, 05:06 PM
Why would you write a script to measure a (3) shot group when Benchrest(capital"B") involves (5) or (10) shot groups?? Just curious. And they are measured center to center of the widest (2) shots.

Mike Swartz

Xfactor
09-09-2008, 10:12 PM
Hi Mike,
The script would do any number of shots - I mentioned 3 only as an example of how the diameter of a circle enclosing a shot "group" (even with just a 3rd shot) can be significantly bigger than the distance b/w the 2 farthest apart shots...
But this is purely academic at this point, b/c now that my question has been answered (and it's been confirmed that Benchrest competitions measure group size simply by the distance b/w the 2 farthest apart shots - center to center), I see no reason to go to a lot of trouble to calculate a true smallest enclosing circle. :D

alinwa
09-09-2008, 10:48 PM
Here's one way to do it

http://www.6mmbr.com/ontargetsoftware.html

Here's a post which shows a picture of the measuring caliper for doing it the more traditional way, the way it's done at matches.

http://www.benchrest.com/forums/showthread.php?t=50209&highlight=OnTarget

I'm sure someone will be able to come up with am image of the device in use if need be.

Both of these methods are more accurate than the enclosing circle method.


al

Cliffy
09-10-2008, 12:04 AM
Do flyers count as part of the group?

Cheechako
09-10-2008, 12:28 AM
xfactor

Maybe I'm missing something here but if you measure a group using the smallest enclosing circle wouldn't the smallest possible result be the diameter of the bullet? And wouldn't it follow that using that method would favor the smaller calibers? :confused:

Ray

alinwa
09-10-2008, 12:31 AM
Do flyers count as part of the group?

On THIS board there is no such thing as a "cried flier", nor a called flier, nor ANYTHING except the whole group, all 5 shots. :D

Although fliers will make one cry!


LOL


al

Xfactor
09-10-2008, 02:40 PM
xfactor

Maybe I'm missing something here but if you measure a group using the smallest enclosing circle wouldn't the smallest possible result be the diameter of the bullet? And wouldn't it follow that using that method would favor the smaller calibers? :confused:

Ray
Hi Ray,
The shots would be logged as x/y coordinates based on the center of the hole, and the circle would mathematically enclose the coordinates (not the holes on the paper) so it wouldn't matter what caliber the shots were... 2 shots through the same hole would be a 0.00" group.

mike in co
09-10-2008, 04:36 PM
well this is where theory and the real world colide.....holes do matter, the entire bullet diameter hits the target, not just the theoretical mid point.

sorry you are working on the shooting sport to be talking theory.

no real world application in this sport.

mike in co

Xfactor
09-11-2008, 09:48 AM
...holes do matter, the entire bullet diameter hits the target, not just the theoretical mid point...

Please explain... if the shot groups are measured by taking the distance between the center points of the 2 farthest apart holes, how does the size of the hole matter?

Joe
09-11-2008, 10:09 AM
Measure the two holes that are the farthest apart, outside edge to outside edge.
Subtract the bullet diameter.
This will give you the center to center distance.

alinwa
09-11-2008, 02:25 PM
Measure the two holes that are the farthest apart, outside edge to outside edge.
Subtract the bullet diameter.
This will give you the center to center distance.


Wish it were this easy :)

Problem is Joe that the hole in the paper never equals bullet diameter. Subtracting .220 for .243 bullets is somewhat close when you're using actual target paper solidly backed. Typing paper and random backer??? All bets are off.

al

mike in co
09-11-2008, 07:53 PM
Please explain... if the shot groups are measured by taking the distance between the center points of the 2 farthest apart holes, how does the size of the hole matter?


let me make thes very clear.

when there is a hole, there is no center point to measure from.

measuring is done with a couple of tools which considers the hole diameter, the distance to the furthest hole diameter, with the bullet dia removed to produce a number which some call center to center, even tho the centers are NEVER measured....its just the group size.


please note the title of this thread...your title....."how are groups measured", not how to measure center to center.

i believe hovis is in error when he posted center to center.

mik ein co

Xfactor
09-11-2008, 11:19 PM
...when there is a hole, there is no center point to measure from....
measuring is done with a couple of tools which considers the hole diameter, the distance to the furthest hole diameter, with the bullet dia removed to produce a number which some call center to center, even tho the centers are NEVER measured...

That sounds an awful lot like a very precise way of measuring the center of the shot... :D
Mike, I think we can all agree that however the center of the hole is determined, the outermost bullet holes in the group are each converted to a center point that has no diameter... and it's those points that are used to calculate the group size.



...
please note the title of this thread...your title....."how are groups measured", not how to measure center to center.
And by the way, the question about how the center of a shot is measured actually wasn't posed by me... but as the thread starter, I think it is certainly a relevant component of my original question, so let's not be so quick to try to railroad a broadening of this discussion, OK?

mike in co
09-12-2008, 12:11 AM
That sounds an awful lot like a very precise way of measuring the center of the shot... :D
Mike, I think we can all agree that however the center of the hole is determined, the outermost bullet holes in the group are each converted to a center point that has no diameter... and it's those points that are used to calculate the group size.



And by the way, the question about how the center of a shot is measured actually wasn't posed by me... but as the thread starter, I think it is certainly a relevant component of my original question, so let's not be so quick to try to railroad a broadening of this discussion, OK?

duh
no we do not agree.
the center of the hole is NOT DETERMINED.
THE PERIMETER is estimated, as is the perimeter of the second hole.
nowhere is the center determined...which is why one bullet diameter is subtracted from the measurement.
BENCHREST shooting is NOT meassured CENTER to CENTER. the same as AGGS are not measured to 4 decimal places, it is a CALCULATION that produces the results.
I DID NOT SAY YOU ASKED ABOUT CENTER, i said, as you posted, how is GROUP measured. you have been given the answer, and are not listening.
i'll try once more.
a tool is sold by a couple of companies. it is based on bullet DIAMETER. the diameter of two holes are used to measure the group size. this is based on the diameter and distance of the two hole. NOWHERE IN THE MEASUREMENT IS THE CENTER OF THE HOLE USED.
you can continue to read thru your rose colored glasses that you are right, but in the end you are just wasting time, and bandwidth.
your comment, sarcastically, that it is
"That sounds an awful lot like a very precise way of measuring the center of the shot... "
just shows you dont get it....we DO NOT MEASURE the CENTER of the shot.
but the process is how we measure, and it has worked well for many years .
go look at the software demo that is posted above. THAT software is based on hole DIAMETER.....just like in BENCHREST.....

duh
mike in co

Xfactor
09-12-2008, 10:25 AM
duh
no we do not agree.
the center of the hole is NOT DETERMINED.
THE PERIMETER is estimated, as is the perimeter of the second hole.
nowhere is the center determined...which is why one bullet diameter is subtracted from the measurement.
BENCHREST shooting is NOT measured CENTER to CENTER. the same as AGGS are not measured to 4 decimal places, it is a CALCULATION that produces the results.
I DID NOT SAY YOU ASKED ABOUT CENTER, i said, as you posted, how is GROUP measured. you have been given the answer, and are not listening.
i'll try once more.
a tool is sold by a couple of companies. it is based on bullet DIAMETER. the diameter of two holes are used to measure the group size. this is based on the diameter and distance of the two hole. NOWHERE IN THE MEASUREMENT IS THE CENTER OF THE HOLE USED.
you can continue to read thru your rose colored glasses that you are right, but in the end you are just wasting time, and bandwidth.
your comment, sarcastically, that it is
"That sounds an awful lot like a very precise way of measuring the center of the shot... "
just shows you dont get it....we DO NOT MEASURE the CENTER of the shot.
but the process is how we measure, and it has worked well for many years .
go look at the software demo that is posted above. THAT software is based on hole DIAMETER.....just like in BENCHREST.....

duh
mike in co
Mike,
First, please settle down and stop trying to hijack this thread into a silly argument over a semantic nuance.
Second, you can describe it any way you want, but the fact is that the entire point of a group measuring tool (manual or software) is to factor in all the variables (first and foremost, of course, being bullet diameter!) and find the center point of the outlying shot holes from which to measure distance. That's the bottom line. Without determining a theoretical center point of the shots, there would be no way to accurately measure the distance between them that would be agnostic to bullet caliber. Think about this for a moment and you'll get it... Or not :rolleyes:

Again, thanks to all for the helpful responses to my question.

Gene Beggs
09-12-2008, 10:51 AM
In his book, "Extreme Rifle Accuracy" Mike Ratigan has a very good description of how to measure group size. It's not hard or complicated.

Gene Beggs

Montana Pete
09-12-2008, 10:56 AM
I always take the two farthest apart shots. I measure from the inside edge of one of the holes to the outside edge of the other hole.

This is tantamount to measuring the distance center to center.

Cheechako
09-12-2008, 11:35 AM
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

MRL
09-12-2008, 11:44 AM
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.

Mike Swartz
09-12-2008, 12:44 PM
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

Jerry Dailey
09-12-2008, 02:50 PM
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.

mike in co
09-12-2008, 03:13 PM
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

Xfactor
09-12-2008, 11:03 PM
...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.
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.



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.

That's cool, Jerry - thanks for the link.

Mike Swartz
09-13-2008, 12:24 PM
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

alinwa
09-13-2008, 02:32 PM
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

4Mesh
09-13-2008, 03:02 PM
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.

Xfactor
09-14-2008, 10:41 PM
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! :) )


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).

Set-Trigger
09-14-2008, 11:03 PM
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