I’m getting confused trying to compare my results with what gets reported by way of group sizes. When someone says they got a 2-inch group, does that mean 2 inches was the between the two furthest apart shots, or something else? Looking at Ballistipedia, it seems group size is a term that could be applied to a whole variety of measurements.
Maybe there’s an unspoken convention here that I’ve missed?
Thanks
Steve
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Traditionally a 'group' is measured between the two furthest apart holes from the centre of each hole. Picture two pins in the target and measure how far apart they are.
Cheers from New Zealand
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.jpg?width=20 "Dukem")
There's also a question of how many shots in the group. Far as I'm concerned, these phenomenal 3 round groups I see in lots of gun mags don't tell me anything more than somebody got lucky. As far as I'm concerned, "group size" doesn't tell me much about any given load until it means the average of at least three 5 shot groups..
How about it? Any comments otherwise?
Wes
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As above. You not only have to be able to do it, you have to be able to repeat it consistently.
John Carlson. CBA Director of Military Competition.
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The "Math Geeks" tell me three 7-shot groups. Myself, I like two 10-shot groups, or one 25-shot group for those outliers. NRA says five 5-shot groups, although they are now shooting lots of three shot groups in their articles.
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In common usage the terms are used interchangeably and they are not:
Many shooters report the smallest group ever shot by their rifle and load claim it as the "accuracy" where is is the smallest example of "precision:" Few shooters have the engineering or science background to have used statistics to understand the difference. And with the common shooter acceptance, the term "accuracy" is useful and provides a common reference for discussion.
The size of an example determines the accuracy. My choice of accuracy testing is five targets used as below; This will take several hours and is intensive.
Keep records of each shot with the wind conditions. This is best shot on the range using wind flags to help you understand the wind's influence.
I shoot ten-shot groups to select the loads to test in the above manner.
Farm boy from Illinois, living in the magical Pacific Northwest
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For serious testing, four, five-shot groups, each on different days, each shot marked in the order in which it was fired, overlayed to make a 20-shot composite, is what I try to maintain for any sort of meaningful test. Conclusions will not be drawn until that process is repeated through a wide range of temperatures.
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I gave up recording extreme spread because so often I’d have a really tight group of maybe five shots, and then the sixth would be a bit of a flyer, and instantly my group size doubled. Eventually I stuck with the average shot distance from the group centre, which I understand is known as the Mean Radius.
Maybe with some basic data, a statistician could advise on just how many shots to a group would give just what level of confidence for results? I likewise think the order of the shots could be important (the “fouling shot” etc.), and sometimes I’ve even wondered about which chamber of a revolver.
The Mean Radius allows me to compare my own reloads, but if the group sizes that people are quoting (mostly, at any rate) really are extreme spreads, then I have to admit they’re in a league beyond me, even with a bench vise.
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When I find a load with potential. I load 40rds. Shot 2 5 shot groups at the start of the day & 2 5 shot groups at the end of the day. Repeat at next range session.
If groups are consistent for all 10rd sessions then I work a bit more to double check. If it maintains that consistent accuracy then I keep a box or 2 on hand.
But I haven't had a new gun to work up a load for in so long, I don't have a need for any new load development. My Rifle/Loads perform as well as I can shoot them.
Long time Caster/Reloader, Getting back into it after almost 10yrs. Life Member NRA 40+yrs, Life S.A.S.S. #375. Does this mean a description of me as a fumble-fingered knuckle-draggin' baboon. I also drool in my sleep. I firmly believe that true happiness is a warm gun. Did I mention how much I HATE auto-correct on this blasted tablet.
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There's also a question of how many shots in the group. Far as I'm concerned, these phenomenal 3 round groups I see in lots of gun mags don't tell me anything more than somebody got lucky. As far as I'm concerned, "group size" doesn't tell me much about any given load until it means the average of at least three 5 shot groups..
How about it? Any comments otherwise?
Wes
We see examples of 5 shot, 10 shot and 25 shot groups....and multiples of these. I don't subscribe to any regimen as such. Try any of those over a year in all the various weather conditions that comes your way..... what does it do as an average then? All the reports of extensive testing mean little until it's done over a long period of time..... one or two trips to the range don't mean much.
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Over extended time periods and numerous groups fired I have come to the conclusion that using the one shot group always yields the most consistent results regardless of conditions.
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"Maybe with some basic data, a statistician could advise on just how many shots to a group would give just what level of confidence for results? "
Yes statisticians can do that. You can do it too. Joe Brennan's book explains how to do it better than most statistic books. The book is on sale by the CBA. See inside the back cover of any Fouling Shot or our website. The book is also available on line.
John
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...some old farmer told me ... " if ya keep shooting, small groups will get bigger but big groups won't get smaller ... "
what statistical rule applies here ?? i think it means that if you shoot 3 shots and every bullet hole is 2 or 3 inches from the others .... you can quit shooting now ... what level of validity have we achieved ?? ...
ken, statistically not sure of anything ... 
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One winter day a couple of years ago I compared the 5 shot groups vs the 10 shot groups for many of the competitors at the 3 years previous National Matches. I don't recall if I included every class of rifle from every year but with 40-50 entrants per year and both 100 and 200 yard groups, it was a respectable amount of data. The ratio was determined by dividing the 10 shot agg (average of 2 groups) by the 5 shot agg (the average of 4 groups) for each participant at each distance. The ratio was just over 1.3. One year had bad weather on the day of the 200 yard match which created a little noise in the numbers but overall, the ratio was fairly steady.
Indeed Ken if ya keep shooting, small groups will get bigger.
Jim
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...some old farmer told me ... " if ya keep shooting, small groups will get bigger but big groups won't get smaller ... "
I shoot a lot of groups, and I'll tell you that sometimes, groups get smaller. A big 2 shot group can become a small 5 shot group .Nyet?
what statistical rule applies here ?? i think it means that if you shoot 3 shots and every bullet hole is 2 or 3 inches from the others .... you can quit shooting now ... what level of validity have we achieved ?? ...
ken, statistically not sure of anything ...
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One winter day a couple of years ago I compared the 5 shot groups vs the 10 shot groups for many of the competitors at the 3 years previous National Matches. I don't recall if I included every class of rifle from every year but with 40-50 entrants per year and both 100 and 200 yard groups, it was a respectable amount of data. The ratio was determined by dividing the 10 shot agg (average of 2 groups) by the 5 shot agg (the average of 4 groups) for each participant at each distance. The ratio was just over 1.3. One year had bad weather on the day of the 200 yard match which created a little noise in the numbers but overall, the ratio was fairly steady.
Indeed Ken if ya keep shooting, small groups will get bigger.
Jim
7.15.7 HOW MUCH DOES GROUP SIZE CHANGE AS RANGE CHANGES?
5 shot 100 yard groups are .45 times as large as 5 shot 200 yard groups.
5 shot 200 yard groups are 2.2 times as large as 5 shot 100 yard groups.
10 shot 100 yard groups are .43 times as large as 10 shot 200 yard groups.
10 shot 200 yard groups are 2.35 times as large as 10 shot 100 yard groups.
On average
See “ACCURACY 100 YARD VS 200 YARD”, in the excel workbooks.
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Joe
These numbers are very very close to the ratios I calculated using National Match data.
JIm
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I think that the amount of recoil can also have an effect, especially over a long string. I helped a friend do some load work for a 338 Win Mag with 250 grain jacketed bullets (Berger Classic Hunter) using Berger's reloading data. Even with a Muzzle Brake and heavy duty slip on recoil pad, it got to the point after five well spaced (time in between) shots, pulling the trigger was not fun.
I notice this effect using cast in a M1903A3. With Lee's version of the Ed Harris SKS bullet, and 16 grains of AL 2400, I am more comfortable vs 25 grains of H4895 and a Lyman 314299 (no filler in either). I almost always see smaller groups (10 shots slow fire, with a sling 100 yard CMP matches) with the smaller lighter bullet. I never see the 160 grain bullet in CBA match results so I doubt it is a more accurate bullet but in the one application I use it for, it is.
Dave
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Steve groups are traditionally measured center to center of the farthest holes in the paper and that's what people expect. The thing you have to realize about comparing your results against those quoted on the internet Is a lot of people follow the "at least 50% rule" which is the results they post on the internet are at least 50% smaller than what was shot on the target. There's also the "tunnel vision phenomenum" where people center in on the few holes that landed close together while ignoring the ones scattered all over the paper. Then there's the "one group wonders" who love to post that one group out of hundreds that just managed to land somewhere in the same vicinity. Just enjoy what you're doing and take a lot of what you read with a grain of salt. This is why I like Joe Brennan's posts. He usually posts multiple groups when delving into something.
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I use to use five shot groups and shoot several then see what the agg. was of the total groups. Lately I have switched to ten shot group, shoot several and then go for the aggregate of the groups. We all know wind conditions, etc. can affect group size. The one thing we don't like to mention is shooter ability, we all have those good days and then those bad days. I have a friend in the fine State of Tenn. that is one of those phenomenal shooters. I see him once a year and we get together for a day of shooting. When I want to see what a rifle is capable of I let him shoot a few groups with it for comparison in my ability. You all have a good day.
Bob
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Over extended time periods and numerous groups fired I have come to the conclusion that using the one shot group always yields the most consistent results regardless of conditions.
I'm with you on that; always get a one hole group, plus it saves on components and time. Makes me feel good about my loading Q and A and rifleman skills.
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jeff houck
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Aaron
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Tom Acheson
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Wilderness
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John Carlson
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sghart3578
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Spindrift
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alphabrass
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SkinnerD
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2frogs
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