standard deviation

Discussion in 'Reloading' started by dskd, Jun 11, 2008.

  1. dskd

    dskd Well-Known Member

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    Mar 20, 2008
    What must be done to get a standard deviation of less than 10 fps ..... ?

    What is your acceptable SD?

    What SD is considered maximum for competitive target rounds?

    What SD's are accepted as competitive by prescision benchrest shooters?

    rounds are 308 and 338lm
     
  2. goodgrouper

    goodgrouper Well-Known Member

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    Q#1. Everything and nothing. Having brass prepped perfectly, the powder trickled to the .1 grain, perfect neck tension, primers seated consistently, and a myriad of other things certainly help get low SD's but sometimes, just having the right amount of the right powder in the case with nothing else done at all will yeild small SD's.

    Q#2. 10 fps or less.

    Q#3 and 4. Same as Q#2 except for close range BR. In that sport, we don't care if it is 100 fps as long as the group is tight.
     

  3. Winchester 69

    Winchester 69 Well-Known Member

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    Glen Zediker in his "Handloading for Competition" has a chapter on the subject, but the most meaningful thing he has to say about it is summarized thusly:

    " A tolerable sd is 12. Anything more than that is not good; anything less than that probably doesn't perform any, if at all, better than a 12."

    His rule of thumb.
    .
     
  4. JeffVN

    JeffVN Well-Known Member

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    Without delving too far into the realm of statisitics, standard deviation is a predictive statment (forgive the gross generalization here) about the velocity spread of your particular load. The higher the standard deviation, the higher the predicted velocity spread (again grossly generalized) of your particular load. The tighter the load, the lower the SD; hence the quest for a tight load with low extreme spread and low SD.

    At long range (especially at 1K and beyond), velocity swings and spreads between shots can have a significant verticle changes at the target end.

    JeffVN
     
  5. overbore

    overbore Well-Known Member

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    Oct 14, 2004
    Agreed

    with all of you and add that the total SD, in my experience, is the product of all the small items subject to variables such as the primer itself, charge weight, case capacity, neck tension etc. Overbore