SD's and ES's??????? Help!!

Discussion in 'Reloading' started by Elkslayer1, Sep 29, 2010.

  1. Elkslayer1

    Elkslayer1 Well-Known Member

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    Ok,

    I hear alot of talk about Ds's and Es's. I have a chronograph but use it just to get the speed of my best round to order turrets.

    Can you geys give me the ins and outs of Sd's and Es's.

    Ps....most of my guns shoot MOA out to 700 yards.

    Thanks!
     
  2. SBruce

    SBruce Well-Known Member

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    It's ES or extreme spread that makes the difference. ES is the difference between the maximum velocity in a particular group of shots vs the minumum velocity in the same group of shots. Too much ES and you will get vetical stringing of your groups at long distance. Too much ES will also make the wind affect your shots differently at the same distance.

    Let's say you're shooting at 900 yds. you've got a 15 mph wind blowing from 04:30. and your loads have got an ES of 75 feet per second. (We'll assume you're shooting a 6 Long Dasher and a .48 BC bullet, cause I've got that loaded into my program)

    If you happen to shoot the extreme ends of your velocity for your first two shots (entirely possible) then the first one would impact over 8" different vertical and 2" different horizontal from the 2nd shot............just because the difference in the velocity of round 1 vs round 2. This is ONLY taking into account the delta V, not cold bore vs warm bore or normal variation in point of impact due to alot of different things.

    Now, lets take the same situation and use an ES of 20 feet per second. Your difference in point of impact for the same two shots would be only 2" vertical and 1/2" horizontal.

    SD or Standard Deviation is much harder to explain and calculate. It is basically a Statisticians argument and takes alot of shots over a very accurate chronograph to accurately determine. I wouldn't worry about it.

    If you only plan on shooting out to 700 yds, then the differences quoted above would be much much less. If your 1 moa groups at 700 yds are nice and round, and don't have more vertical than horizontal........you're ES's are probably ok.............just depends on how far you're gonna shoot and what size targets you plan on shooting at.

    Hope this answers your Q's.
     
  3. Elkslayer1

    Elkslayer1 Well-Known Member

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    SBruce thanks for your reply!!!!

    How many shots would you consider a good number?

    What are good E's ranges for 7MM and 30 Cal rifles?

    Thanks
     
  4. phorwath

    phorwath Well-Known Member

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    Most all can readily understand average MV and Extreme Spreads in measured muzzle velocity. But how does one explain the uniformity or the dispersion of MV? If the ES is 50fps, how many bullets out of 100 fired leave the muzzle with velocity that varies by 10 fps, 20 fps, 30 fps, or 40 fps?

    That's what Standard Deviation (SD) provides. It's a measure of the uniformity of your MVs. Most chronographs will calculate SD these days. If you have an ES of 50, you don't know how many bullets out of 100 fired will leave the muzzle with that difference of 50 fps. If you shoot over your chronograph during load development, by the time you're done, you'll probably have enough rounds fired to determine a reasonably correct SD. An SD of 15 means that 2/3's of your bullets will leave the muzzle within 15 fps of your average (mean) MV.

    Example using this data set:
    Average MV = 3000 fps. ES = 30 fps. SD = 10 fps.
    The SD of 10 means that 2/3 of your shots should be expected to have a MV between 2990 and 3010 fps.
    Which also means that 1/6th of your shots should be expected to have a muzzle velocity between 2895 and 2990 fps.
    And the remaining 1/6 of your shots should be expected to have a muzzle velocity between 3010 and 3015 fps.

    I can usually get SDs down to 15 fps or less with some load development time and my goal is an SD of 10 or less. Any SD of less than 10 fps is pretty darn good for a large capacity rifle cartridge like the 7mm Rem Mag or 300 Win Mag. The smaller the better - same as with group measurements. However many consider any single digit SD to indicate a very acceptably uniform load with 7mm Rem Mag and 300 Win Mag class cartridges for purposes of shooting long range (~1000 yds).
     
  5. J E Custom

    J E Custom Well-Known Member

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    +1

    Very well said.

    The lower the number the the more consistant the rifle/load is shot to shot.

    J E CUSTOM
     
  6. SBruce

    SBruce Well-Known Member

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    Phorwath,

    Thanks for the info. That clears up the SD's some and makes alot more sense than the prior explanations that I'd heard!

    How many shots does it take to get a reliable and "real" SD?? is it 100??

    Is a shooting chrony a good enough instrument to get a true SD? Or does it take alot more expensive (and accurate) chronograph, like an Ohler 35P or similar?..........Garbage in, Garbage out type of thing.

    Some will argue to the death saying that a 3 or 5 shot group isn't enough to really determine average group size. The same argument (I am assuming) would apply to the SD's given by the chronograph if only 10 or 20 shots are fired through one??

    If I measure the length of 20 fence posts out of a pile of 200 and figure the SD based only on those 20, wouldn't it be entirely possible to be an incorrect value.?
     
    Last edited: Sep 30, 2010
  7. phorwath

    phorwath Well-Known Member

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    FYI: I am not a statistician. I'm sharing some thoughts and my opinions based on my knowledge, and experiences. Good shooting to ya.
     
    Last edited: Oct 2, 2010
  8. SBruce

    SBruce Well-Known Member

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    You're Welcome, but I really can't answer about 7mm and 30's because so far, all of my long range and benchrest experience is with 22's, 6mm's and 25 calibers.

    Kirby Allen is building me a 300 WSM as we speak though.:D

    With the smaller calibers and way way lower BC's, I've always tried to keep the ES under 30 fps (not often successfully), measured over at least 10 rounds, but usually more like 30 rounds.......I figured that ES under 30 automatically and generally means a low SD also (I never had a chrony that would calculate SD, so I had to do it manually if I wanted that info)

    However, if I was getting good groups at extended ranges, I never even bothered with it; I just shot the guns at the ranges I planned on using them at, recorded POI and conditions, and had fun. I figured that 1/2 moa under good conditions, off the bench was plenty good for hunting purposes........better than I can shoot under hunting/field positions and conditions for sure, and I knew for a fact where they were shooting under different temps/conditions that way too.

    Phorwath had given a great explanation of SD's however, and I am assuming that has answered your original Q's??
     
    Last edited: Oct 2, 2010
  9. SBruce

    SBruce Well-Known Member

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    Phorwath,

    Thanks again for the detailed explanation of SD's and the answers to the Q's.

    Much Appreciated.