How are shot groups measured?

X

Xfactor

Guest
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
 
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?! ;)
 
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
 
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
 
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
 
Cheechako said:
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
Click to expand...
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.
 
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
 
mike in co said:
...holes do matter, the entire bullet diameter hits the target, not just the theoretical mid point...
Click to expand...
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?
 
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.
 
Joe said:
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.
Click to expand...


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
 
Xfactor said:
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?
Click to expand...


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
 
mike in co said:
...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...
Click to expand...
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.


mike in co said:
...
please note the title of this thread...your title....."how are groups measured", not how to measure center to center.
Click to expand...
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?
 
Xfactor said:
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?
Click to expand...

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
 
mike in co said:
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
Click to expand...
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.
 
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
 
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.
 
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