Roc Doc
2A SNBI
This from Bryan Litz -
One thing you learn when you shoot a lot of groups is the wide range of group sizes that are normal.
Many shooters seem to expect a gun to shoot basically the same group size all the time, but that's simply not how it works. What we've seen over 100's of 5-shot groups from .223 thru .375 is that the standard deviatino of 5-shot groups is about 30% of the average.
Statistics tells us that 67% of groups will be between +/-1 SD of the average, and 95% of groups will be within +/-2 SD's of the average.
So for example, if you're long term group average is 0.5 MOA, then 67% of your groups will be between 0.35 and 0.65 MOA. Likewise, 95% (19/20) groups will be between 0.2 and 0.8 MOA. Anything in that range is completely normal for a 0.5 MOA average.
This is the nature of dispersion.
For this reason, it actually takes a lot of 5-shot groups to accurately characterize the true average precision. We consider 5-shot groups decent, but sometimes it takes ten 5-shot groups to resolve a genuine precision difference in an A vs B type test.
Remember this next time you're doing load development and call one load 'better' than another because it shot a 0.5 vs. 0.6 MOA group. Those single samples are more likely to be the same than different.
Follow me for more shooting statistics tips
Attached picture; each row is 10 groups of the same rifle/ammo combo.
One thing you learn when you shoot a lot of groups is the wide range of group sizes that are normal.
Many shooters seem to expect a gun to shoot basically the same group size all the time, but that's simply not how it works. What we've seen over 100's of 5-shot groups from .223 thru .375 is that the standard deviatino of 5-shot groups is about 30% of the average.
Statistics tells us that 67% of groups will be between +/-1 SD of the average, and 95% of groups will be within +/-2 SD's of the average.
So for example, if you're long term group average is 0.5 MOA, then 67% of your groups will be between 0.35 and 0.65 MOA. Likewise, 95% (19/20) groups will be between 0.2 and 0.8 MOA. Anything in that range is completely normal for a 0.5 MOA average.
This is the nature of dispersion.
For this reason, it actually takes a lot of 5-shot groups to accurately characterize the true average precision. We consider 5-shot groups decent, but sometimes it takes ten 5-shot groups to resolve a genuine precision difference in an A vs B type test.
Remember this next time you're doing load development and call one load 'better' than another because it shot a 0.5 vs. 0.6 MOA group. Those single samples are more likely to be the same than different.
Follow me for more shooting statistics tips
Attached picture; each row is 10 groups of the same rifle/ammo combo.