How are groups measured in competition?

[JustC]

I have heard many different things from many people, and I would like to know how to accurately tell group size.

I have been under the impression that the center of the two farthest apart holes was the determining measurement. is this correct? If not, what is the "correct" way?

thanks

[Brent]

True, center of the two measured from outside of one hole to the inside of the opposite hole is the same as the center to center but you have an edge to work with.

Measure outside to outside when groups are tight and subtract the bullet diameter, this will give you the same too.

[4mesh063]

If i'm right, what you were meaning was do they take the farthest 2 shots or do they take a smallest triangle of 3 possible shots. Even after a year of shooting I still don't know that answer. I would say to ask Darrel Cassel since He's been shooting 1000yd competition since WW1 [img]images/icons/wink.gif[/img]- I didn't have any groups through the year that 2 shots didn't define so I never knew if they use the smallest circle which would cover all the shots. If they use 2, instead of 3, I would say that the smaller the groups get the more error that will be inherant to the measuring process. I'll have to ask about this. 3 would almost require Autocad to plot though. That sounds like too much trouble to be worth it.

[Brent]

If three shots form a triangle the farthest 2 apart are measured, or the farthest two shots apart whatever shape they form. This is the only method I've ever heard of that is used.

[Steve Shelp]

Brent is correct on this and all shots are counted/measured. Here's a extreme example to show how it's done for clarity.....

If you shoot a 10 shot group with 8 rounds going directly through the center of the X ring and the 9th shot went 6" directly to the left and the 10th shot went 6" directly to the right you would have approx a 12" group. Calipers with extentensions and optical and plugging devices are used to accuractly tell the center of the bullet holes to get fairly accurate results. But it's still not an exact science. You can take a target and have 5 different people measure it and you might get 5 different measurements that vary by a couple of thousands. Depending on the paper, humidity, backer material, etc different caliber holes measure differently also if you want to get down the the nitty-gritty of measuring. A 28 caliber hole in one paper target may measure .275 in diameter and on another it may measure .273. So when you subtract the caliber diameter you must take into account the actual caliber hole size on that day if your measuring outside to outside like Brent described above to be exact.

But to keep it simple when measuring groups for my hunting guns or informal testing I use the outside to outside measurement and subtract the actual caliber diameter and forget about the few thousands difference in the hole size. If all of my targets have the same error in the same direction, I can tell the better groups from the others and that's all I need to know, so it's irrelevant.

Hope this clears things up.

Steve

[ 12-11-2002: Message edited by: Steve Shelp ]