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Thread: H322 and 77 grain Sierra TMKs.

  1. #41
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    Quote Originally Posted by lsllc View Post
    But what does the SD tell you that is useful? It’s irrelevant information. Knowing it doesn’t help your load.


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    I'm sure there are some guys here that know way more about statistics than I, but I'll take a stab at it an explain why I look at SD and rarely at ES during load development.

    Standard Deviation will tell you the likely-hood of a shot's velocity falling within a certain deviation from the mean velocity. The Extreme Spread only gives you the difference between the two outliers in your string. A round that I may have dicked up could be the cause for a high ES, while while a high SD is indicative of a load that will consistently result in a larger vertical divergence at distance.

    For a visual reverence think of a bell curve with steep sides having a low SD (the vast majority of the shots are in the center by the mean).
    The high SD bell curve is one that looks like it got pancaked with a hammer. Yeah, all the shots are still within the same ES, but they're as likely to be on the high and low end as they are in the middle.

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  2. #42
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    I get that. But the ES is telling you about your process. The ES is telling you not only that your load is adequate, but that you aren’t making mistakes in your loading process. Those outliers are misses. The 90% or 95% within your deviation just tells you that the load works. But it’s a poor indicator of when you’re going to miss because of your ammo.

    Much of the time when dudes blame wind, they are under or over the target because they had a round 50 FPS from their average velocity. Their process needs fixed or their components have a problem. But if the guy doesn’t look at ES, and only looks at SD, their SD will indicate there isn’t a problem and it’s just an “outlier”. When the guy works up the load an only runs ten across the chronograph, one or two rounds are “outliers”, what happens when ten or twenty percent of the time 100 rounds they loaded for a match or for hunting turn out to be “outliers”? They MISS. They either drop one into the 7 ring at 600, or miss 2/3 IPSC targets at 800, or they gut shoot an animal.

    If loading for precision, the “outlier” needs to be within 20 FPS, but preferably 10 FPS.


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  3. #43
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    Think of it this way, you can’t have a bad SD with a good ES. But you can have a good SD with a shit ES.


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