Measuring group size

[Jim]

As I understand it, group size is measured across the widest part of the group. Is the proper measurement across the outside of the bullet holes or center to center?

[btroj]

I measure center to center. Actually I determine the two farthest apart holes. I measure from the right side of one hole to the right side of the other. This gives same as center to center but allows me to measure from a fixed point rather than guesstimating the center of a hole.

I hope that makes sense.

[Jim]

That makes perfect sense, btroj. That's the way I've always done it. The reason I asked is because I saw a photo here on the forum of a caliper across the outsides of a group and it got me to wonderin' if I was measuring correctly.

Thanks!

[357shooter]

Center to center is the correct measurement. Left side to left side, or the other way, makes it easier to take the same measurement.

For very tight groups, you can measure the outside, find a standalone hole, measure and subtract it. That will give you the center to center. Don't use the bullet diameter, instead use the actual size measured in the paper.

Added later: Wow, you guys posted while I was typing!

[C.F.Plinker]

I set my caliper to the diameter of the bullet then zero it out. Measurements are from outside to outside of the two shots furthest apart. The caliper does the subtraction and gives the center to center distance. With lead boolits you can often find a shot where you can see leaded circle the boolit made as it went through the target. If this is the case measure it and then zero out the caliper.

For quick use at the range I have a 6 inch scale. I taped a piece of cardboard on the "zero" end and marked the diameter of various calibers on that going away from zero. All I do is to place the caliber mark on one side of the group and measure to the other side. The resulting measurement is also center to center.

[Trey45]

I measure outside to outside across the widest point and subtract bullet size. In this case I have a 10 shot group that measures 1.858, from which I subtract .430, my bullet size. Which gives me a group size of 1.428
http://images14.fotki.com/v373/photos/6/1809686/10101988/JS013-vi.jpg

[44man]

When I was young I measured outside to outside so a half inch group was actually smaller. It was a long time before I found out it should be center to center. Never bothered me a bit!

[camaro1st]

center to center?:shock:now i'll have to tape 2 yard sticks together. :groner::bigsmyl2:

[stubshaft]

CTC the same way that Trey does it.

[PS Paul]

Unless actually shooting very accurate rifles, I usually just measure outside to outside without subtracting boolit diameter.

If shooting for groups with "better than average"accurate rifles, I then will measure from outside to outside and subtract boolit diameter OR diameter of holes, depending upon the boolit "style". If a blunt nose style or levergun style that cuts a nice, clean hole, then full-diamter of projectile, kinda like Trey45 does...... I think with pistols, the method I use keeps me from exaggerating my abilities. LOL!!!

[bob208]

droup sixe is center to center or how the other calulate it but still it ends up center to center. that comes from the old string measure where you measure from the center of the target and the center of the holes and add up the total. the one with the less string wins. in muzzleloading matches all scoring is to the center. that cuts out the big bore advantage.

in centerfire matches if a shot is toching a scoreing ring. it receives the higher score. but then you are not worried about groups as much as score.

hope i did not muddy it up for you.

[Blacksmith]

I just shoot the biggest clanger I can find then "I don't need no stinking ruler.":kidding:

[gray wolf]

I measure outside to outside across the widest point and subtract bullet size.

[DCM]

I usually find it easiest to measure outside to outside and subtract bullet diameter also.

For those who like computer toys you may want to try this. http://www.ontargetshooting.com/index.html

I have never used it personally but have seen others use it, seems to work well.

[Smitty's Retired]

+1 on what Trey said. That is about the most accurate for a quick reading.