MEASURING A GROUP

[ANDY T]

Hi i will be running a inter-club .22 rim-fire 75mts competition that will consist of two groups of 5 shots per shooter, with the smallest of the two groups to count.
Then the smallest three groups shot by there best three shooters go forward as that clubs team score.
The individual groups will be measured then added together then divided by three giving a average the team score with the lowest average wins.
My question is what is the best and most accurate way to measure a group

Thanks
Andy

 

[RonAKA]

There may be some competition specific method. But the one I have used in fun competitions is to shoot the groups, plus 4 separate calibration shots. You use a 6" vernier gauge to measure the outside diameter of each of the calibration shots (circled on the target), and average them to get the hole size. Then you measure outside to outside of each group using the same method for what is the edge of the hole on the calibration shots. I measure to the outermost black smudge. From each group you then subtract the average calibration shot diameter. The target below may make it more obvious.

 

[necchi]

Yup, outside to outside of the furthest apart shot's, then subtract the caliber.
The result is like measuring center to center.
Put a hand behind the paper and carefully press the paper back into the hole to see the black smudge of the bullet hole.

 

[LHSmith]

What kind of targets? The reason I ask is the big organizations (IBS, NBRSA) use targets made from top quality paper which resist tearing and helps make the bullet holes more definitive. They also replace the backers frequently. Even with these targets and special group measuring devices it is interesting to find out that if 3 people measure a group, more than likely you'll get 3 sets of numbers....not necessarily even close. Being an official scorer will be a thankless job.

 

[RonAKA]

Yup, outside to outside of the furthest apart shot's, then subtract the caliber.

Actually there is a subtle difference to the method I described. You do not subtract the nominal caliber. You subtract the average single hole size, measured by the same method as you measure the group.

In the target I posted those were .22 LR bullets, but I was only subtracting .206, not .223. If you subtract the actual caliber it underestimates the actual group size. In my example if I subtracted .223 from a group where all the bullets went into a single hole, the resulting group would be a negative 0.017"! Probably hard to get anyone to believe that one...

 

[RonAKA]

There is also a computer program that can be used to measure the targets. I've never tried it, as it seems like a pain to have to scan or photograph each target.

On Target Software

 

[Jeff A]

If I am doing general measuring, I use a Caliper and subtract the bullet diameter. If I want to be precise, I use the On Target Software.