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Dispersion

Roc Doc

2A SNBI
Joined
May 20, 2020
Member Number
580
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2,936
Loc
Way out West.
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.
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Attached picture; each row is 10 groups of the same rifle/ammo combo.

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I've read his books and have heard people refer to him as 10lb brain Bryan. I've spoken to him briefly at Shot and he is scary smart and constantly pushing the science of ballistics. I wish they'd bring their fancy dopler rig to AZ, maybe at a big match in winter.
 
Sometimes I truly hate load development. Dispersion only adds to the frustration.

I use the Applied Ballistics program and have noticed with at least with 2 rifles (different calibers) that around 700 yds. the calculator has me dialing too much elevation. I wish there was more time to resolve these issues.
 
Sometimes I truly hate load development. Dispersion only adds to the frustration.

I use the Applied Ballistics program and have noticed with at least with 2 rifles (different calibers) that around 700 yds. the calculator has me dialing too much elevation. I wish there was more time to resolve these issues.

The problem I run into is that to do MV calibration you're supposed to do it at a target where the bullet will be about 1.2mach, yeah, that's 1600 yards for my 109g GT load. At that distance the mirage is so bad most of the time the target is dancing around. I've got some smaller targets at just under 700 so I adjust till I get hits, take it out to 1k and back down to 200 or so. If I can get hits on all those targets, that's good enough for what I do.

I was reading a thread on The Hide about moisture content in powder and how to make it consistent, etc., and this one dude posts up, "Why do I do this again". :laughing:
 
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