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Group Measurement's Proper Criteria

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Post by brassmaster on 9/18/2018, 8:15 pm

I'm probably plowing old ground here, but just when I seem to have settled---in my own mind, which criteria is proper for measuring and determining a 50 yard group size that is the "Bullseye standard of measurement" using a caliper measurement of outside-to-outside, someone one offers to sell a 1911 with a phenomenal 1"-10 shot, 50 yard group, but measured center to center of the outside bullet holes. Which method is the "accepted standard," center-to-center or outside-to-outside?
Other than sounding more impressive noting the group size utilizing center-to-center measurement, is there any benefit other than the wow factor? Obviously, if a confirmed, knowledgeable and seasoned Bullseye Shooter is prospecting for an accurized or factory produced match-grade gun, a 10 shot, 50 yard target produced by the gun under consideration and an explanation of how group size was determined would seem to be a decider. If one is selling a match-grade 1911, a responsible seller would not want to misrepresent the accuracy, so, identifying (either text or photo) how the group size was measured would seem to be the prudent course of action. Maybe the center-to-center measurement just makes for better bragging? Dunno.
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Post by zanemoseley on 9/18/2018, 8:34 pm

I come from a rifle background, the standard group is a 5 shot group, always measured center to center. A lot of pistol shooters test 10 shot groups. Regardless a group is always measured center to center.

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Post by dronning on 9/18/2018, 8:42 pm

The easiest and most commonly used method of measuring shot groups is the “center-to-center” method.  That way if you are comparing 2 calibers you are stating the group size in a common and equal method.  A .952" outside 45 group and a .720" outside 22 group are really both .500" center to center groups.

I measure outside to outside and subtract the bullet diameter to get center to center measurement.
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Post by cdrt on 9/18/2018, 9:10 pm

dronning wrote:The easiest and most commonly used method of measuring shot groups is the “center-to-center” method.  That way if you are comparing 2 calibers you are stating the group size in a common and equal method.  A .952" outside 45 group and a .720" outside 22 group are really both .500" center to center groups.

I measure outside to outside and subtract the bullet diameter to get center to center measurement.
What he said.
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Post by Chris Miceli on 9/18/2018, 9:13 pm

Eley measures outside to outside on the test range. So winners measure that way
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Post by zanemoseley on 9/18/2018, 9:19 pm

Lol, winners huh. Guess they don't shoot centerfire, I've never seen anyone in serious target shooting measure outside to outside.

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Post by Mike38 on 9/18/2018, 9:29 pm

I've heard of group size measured inside to inside, because that is how you would score a hole. A .45 hole just touching the x ring scores as an X. But if you used the center of the same hole, it would be a 10. Personally, I use center to center.
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Post by jglenn21 on 9/18/2018, 9:34 pm

Chris Miceli wrote:Eley measures outside to outside on the test range. So winners measure that way


and they are probably using the metric system too

Dronning has it right.
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Post by Chris Miceli on 9/18/2018, 9:52 pm

Cjglenn21 wrote:
Chris Miceli wrote:Eley measures outside to outside on the test range. So winners measure that way


and they are probably using the metric system too

Dronning has it right.
Nope imperial 50 shot groups outside to outside
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Post by dapduh2 on 9/18/2018, 10:24 pm

I have a Les Baer in commercial row now. It has a picture of the group with digital caliper sitting on top. I measured center to center at 1.00 inch. If you want to know the outside to outside just add .45 for a .45, or .38 for a .38, etc. I use center to center because of my rifle background as well but it also just makes more sense to me. It keeps it standard across all calibers and I want to know the exact center of where each round went, because that to me is the “zero”
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Post by zanemoseley on 9/18/2018, 10:25 pm

The noob is strong in this thread.

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Post by Wobbley on 9/18/2018, 10:52 pm

Outside to outside makes sense if scoring is “outward”.  In some international systems for rifle scoring is done outward for some values.
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Post by james r chapman on 9/19/2018, 8:07 am

Center to center allows comparison caliber v. Caliber.
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Post by lyman1903 on 9/19/2018, 8:15 am

Mike38 wrote:I've heard of group size measured inside to inside, because that is how you would score a hole. A .45 hole just touching the x ring scores as an X. But if you used the center of the same hole, it would be a 10. Personally, I use center to center.


another rifle background here, (raises hand slowly)

so center to center for group size (as in how the firearm does,  how my reloads do, etc) 

scoring is completely different,
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Post by Slartybartfast on 9/19/2018, 8:39 am

As a "noob", as one less than gracious poster commented, simple search shows outside-to-outside measurements are what you measure. Then you subtract the bullet diameter.
Seem to be a real non-issue. Once you have one of the three possibilities, the other two are easily obtained using the calibre.
Mind you, some people really like quibble about tiny differences in measurement when measuring and comparing measures of sometimes questionable statistical significance.
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Post by james r chapman on 9/19/2018, 8:45 am

So, a .45 outside measured group is only as accurate as the same outside measured .22 group?
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Post by zanemoseley on 9/19/2018, 9:05 am

Slartybartfast wrote:As a "noob", as one less than gracious poster commented, simple search shows outside-to-outside measurements are what you measure. Then you subtract the bullet diameter.
Seem to be a real non-issue. Once you have one of the three possibilities, the other two are easily obtained using the calibre.
Mind you, some people really like quibble about tiny differences in measurement when measuring and comparing measures of sometimes questionable statistical significance.

I'm just giving you guys a hard time, don't get too righteous on me. I'm just trying to help you guys not look like a goober when you talk to someone who is serious about target shooting. Show up at a match where group size is part of scoring and start talking about your awesome inside to inside group and you'll get laughed off the line.

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Post by joy2shoot on 9/19/2018, 10:13 am

james r chapman wrote:Center to center allows comparison caliber v. Caliber.
Showing my ballistic ignorance here, but maybe it also allows comparison between bullet types.  For example, if I have two targets, one shot with .45 full wad cutter and one shot with ball ammo, won't it be easier to compare group sizes using center-to-center?  Measuring outside-to-outside would be easy with full wad cutter, but maybe not so much with ball ammo.

p.s. For whatever its worth, when I watch the Accuracy X videos, it appears Steve is measuring center-to-center.

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Post by james r chapman on 9/19/2018, 2:31 pm

With quarters, dimes and nickels, lol
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