measuring groups?

[Big Tom]

I have been reloading for a couple years now and becoming more fond of bench rest shooting my loads.
My question is; What is the LEGAL method for measuring 100yd groups of the bench? Do you shoot three or five shot groups in competition? I've seen too many different comments on this topic on other forums.

I figure...YOU GUYS SHOULD REALLY KNOW!  :lol:  Thanks! Tom

[longwinters]

I did a search on "group size" and it seems to be the consensus that you measure from center of hole to center of hole at the widest point between bullets holes.  

Long

[Green3845]

Tom-

The easiest, quick and dirty method that doesn't need specialized equipment is to use a dial caliper to measure from the outside edges of the two bullet holes farthest  apart, and then subtract one bullet diameter, which will give you the center to center spread.

Let's say you shoot a five shot group at 100 yards with your  .308.  Your calipers tell you the two bullet holes farthest apart are  .986.  .986-.308=.678 group size.

[Big Tom]

Thanks Guys. I kind of thought that was the way groups were officially measured.

[arky65]

hey guys, thanks for the info. a few questions :

1) why the difference in methods longwriters center to center, green 3845 outside to outside?

2) if you shoot multiple shots into one hole how do you find the center of the different shots?

3) do you measure the the hole or the gray ring around the hole?

[longwinters]

I think Green and I are doing the same thing, (will end up with the same measurement), just going at it a different way.  It may be easier (especially if you have one ragged hole) to use Greens proposed method.

Long

[Green3845]

The method I talked about is center to center. You measure from the outside of the group to the other outside and subtract one bullet diameter from that measurement! One bullet diameter means one half of a bullet diameter on each side, which means center to center!

For non registered benchrest matches, measure from the back ring on the paper. It is the easiest to point to find repeatedly.

The benchrest world has some specialized measuring equipment to be able to discern groups in the "ones"-.10+.

[cal sibley]

The problem in measuring the center of each hole is that it isn't actually there.  What are you measuring?   Empty space?  That's why we measure outer edge to outer edge and subtract one caliber.  It works.  You can also measure one inside edge on one side and the outside edge of the other, then subtract half a caliber.   That also works.  There are several ways to do this.  Best wishes.

Cal - Montreal

[Lone Star]

3) do you measure the the hole or the gray ring around the hole?

You measure the outer grey ring, this is the carbon from the sides of the bullet being rubbed off on the paper target and defines the outer edges of the bullet's passage - or does it?  If you measure the outside-to-outside of the farthest holes and subtract one bullet diameter you assume that the grey ring is actually the diameter of the bullet.  Measure a few rings with digital calipers and a magnifier and you may be surpirsed.....it's better to measure the left edge of the two farthest holes.  This won't work if the edges are shot away in a tiny group.

[jeff]

Sure wish that I could figure how to get what I scan to come out the same size as the original

Anyhow, here's four five shot groups.

[OnAim]

I appreciate the info on this thread... was wondering the samething myself.  I haven't shot much since I was a kid, and back then we scored everything.  My youngest son seems to have the shooting bug, and I have enjoyed "competing" with him at the range.   The idea of shooting sub MOA seems like a good objective... especially to do it consistently...  sounds like center-to-center is the answer.

jeff--- I double clicked on the image you posted, and it was large enough to read.... nice groups!