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Thread: Standard Deviation & Relaoding

  1. #1

    Standard Deviation & Relaoding

    Hi all,

    Earlier this year I investedin a Chrono.
    What I’m curious about is how much notice one should give to Standard Deviation (SD) and what correlation that has to ones grouping.
    As far as I understand its really just a quantifiable measurement of how consistentyour reloading process is.
    I’m really pleased with my SD’s when doing load development for my 6.5x47 but the lowest SD isn’t necessarily the tightest group...


    Over the summer I have worked up a load for the 47 using 120grain Nosler BT (Hunting) and the SD's were between 5.25 and 2.63
    From what I have read, I understand that is pretty acceptable.

    I would be interested to hear your thoughts around the subject.

    Cheers,
    Miles


  2. #2
    I use SD to assess the quality of my brass prep and reloading. I'm pleased if I can get the SD below 10 fps. Regards JCS

  3. #3
    Fram a statistical standpoint, your standard deviation isn't a very useful measure for small numbers of shots. It looks like you've done a very good job to get such small deviations (assuming it's in fps not m/s.
    You can, of course, calculate the correlation yourself and have a definitive answer. If for each group you know the SD and the group size, you can do this on excel, or just get an ALevel student to do it.

    When/if you get large SDs you may be able to investigate whether that was just one wild cartridge or something wrong with that batch. A career in quality control lies ahead ....

  4. #4
    Quote Originally Posted by 2428 miles View Post
    Hi all,

    .
    What Iím curious about is how much notice one should give to Standard Deviation (SD) and what correlation that has to ones grouping.
    As far as I understand its really just a quantifiable measurement of how consistentyour reloading process is.

    Miles

    There is no direct correlation between SD and grouping. Small numbers in the SD column do not mean that a load will be accurate. As you indicated, it is only an indication of how well your ammunition was assembled, consistency of brass, etc. What it your sample size and how are you calculating the SD? Does your chronograph have an SD output as well as extreme spread?~Muir

  5. #5
    To back up what has already been said, when shooting 6mmBR, my tightest groups were not those with the best SD.
    SDs in single figures show that you have a good consistent and refined reloading technique.


    Ian

  6. #6
    +1 to Muir's post - I've had good groups with a large SD (and ES) and vice versa. That was at 100 m though - I know some people think things might be different as the range increases?

  7. #7
    Quote Originally Posted by AndrewS View Post
    +1 to Muir's post - I've had good groups with a large SD (and ES) and vice versa. That was at 100 m though - I know some people think things might be different as the range increases?
    Maybe. With additional range comes changes in velocity, BC and if there are stability issues to begin with (like the bullet is marginally stable for the rifle's twist and MV) these will become apparent down range. I just ordered some 6.5 x 140 grain Nosler "RDF" bullets for my 6.5 Creedmoor. These bullets are a whopping 1.43" long with a stated BC of .658. The first thing I did was to run them through a ballistics program (like Bergers) that will give a stability rating for my proposed load. Were it marginal, I might get great grouping at 100 and 200M but suddenly see a disproportional widening of group size at longer ranges as the velocity and fades and resulting instability increases.~Muir

  8. #8
    Quote Originally Posted by Muir View Post
    What it your sample size and how are you calculating the SD? Does your chronograph have an SD output as well as extreme spread?~Muir
    Hi Muir, thanks very much.

    Yes, my Chrono does indeed give SD and ES. As well as Min, Max and Average.

    Its the Caldwell one that plugs directly into your smartphone so you have all the data at your fingertips. Its excellent!

    Shoot at 100y and in 5 shot groups.

    Thanks everyone for your input.
    Miles

  9. #9
    SD and ES, for most deer stalking ranges of up to a few hundred metres, are pretty irrelevant imho. It comes into its own at extended ranges, 300 yds plus, where velocity differences start to manifest themselves with opening grouping. At 600 yards (target) and further, especially when competition shooting, most aim for as low an SD and ES as possible as at these ranges, it only takes one shot to be a few inches off the rest and miss the V bull and lose a match. Consistency seems to be everything for longer range shooting, but only when someone can learn to read the wind conditions well, especially where they are rapidly changing! ES on it's own may not tell you very much, especially if there's one "flyer" in the group which might otherwise all be within 5 or 10 fps of each other.

  10. #10
    at short range it shows your reloading is consistent and may or may not effect accuracy at longer range starting at about 300m but defiantly after 500m the velocity difference will effect spin rate (rotation) Time of Flight(velocity) and gas bleed off all these will effect accuracy

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