Drag tables

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Guest, Jun 8, 2005.

  1. Guest

    Guest Guest

    When using a Sierra HPBT match king. Would one use a G-5 drag table or the standard G-1. I'm playing around with RSI and infinity RSI offers the option to change drag table.

    Also Pressure?? What is this and what numbers go into that area? I understand altitude Humidity temp etc. Pressure?
    Thanks in advance

    Joe
     
  2. Jimm

    Jimm Writers Guild

    Messages:
    1,270
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    Jun 12, 2007
    Heeeeey Joe, whatcha doing with that gun in yore hand ,da duun da duun da dunn dunn /ubbthreads/images/graemlins/grin.gifSorry Joe , could'nt resist that opportunity. All the recent info I've been reading including that coming from Sierra indicates the G1 drag function is best suited for all applications tthat would apply to the reloader. As always there is probably more to it than that .
    Jim
     

  3. Guest

    Guest Guest

    Works for me /ubbthreads/images/graemlins/smile.gif

    Joe
     
  4. JBM

    JBM Well-Known Member

    Messages:
    113
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    Jan 12, 2004
    Let me see if I can illustrate this. The drag of the bullet (using the ballistic coefficient
    and drag function model) is proportional to the
    drag function divided by the ballistic coefficient (among other things).

    This means as the BC goes up, the drag drops. Since you have two numbers
    to play with, the drag function value and the BC value, there are many
    combinations that give you the same drag value. For example, you can use
    the G1 drag function and a G1 BC or you can use the G7 (or G5, or G8...)
    drag function and a G7 BC. As long as you use the BC developed for the
    drag function you get the right answer -- that's the key. Look at the
    following data calculated for the 210 grain Berger VLD:

    Vel. G7 G1
    1500 0.327 0.590
    1600 0.320 0.600
    1700 0.317 0.612
    1800 0.316 0.621
    1900 0.316 0.626
    2000 0.316 0.630
    2100 0.316 0.633
    2200 0.317 0.636
    2300 0.318 0.638
    2400 0.319 0.640
    2500 0.321 0.643
    2600 0.322 0.647
    2700 0.324 0.651
    2800 0.326 0.656
    2900 0.328 0.663
    3000 0.330 0.670
    3100 0.332 0.677
    3200 0.333 0.686
    3300 0.335 0.695
    3400 0.337 0.705
    3500 0.338 0.715

    It is two ballistic coefficients as a function of velocity for two
    different drag functions. If the drag functions fit the bullet perfectly,
    the number would the same for all velocities -- that's the point of
    the ballistic coefficient -- one number instead of having to remember
    and implement many different coefficients.

    Now when you look at the data above, which coefficient comes closer to the
    ideal, the G7 or the G1? Obviously the G7 fits better because it varies
    from 0.327 to 0.338 (difference of 0.011) compared to the G1 change
    of 0.125, more than 10 times as much.

    Now ask yourselves which BC the bullet manufacturers would rather list.
    Would they rather publish 0.330 or 0.670? Of course they're going to
    publish the 0.670 because it sounds better and many people don't
    understand what it all means. So we're stuck with multiple ballistic
    coefficients as a function of velocity to make up for the short comings
    in the drag function. We could easily use a single number if we used
    the G7 drag function. To make it worse, many programs don't (or didn't)
    allow you to pick your drag function.

    So, if you're using BCs published by Sierra, use the G1 drag function
    or you'll get huge errors.

    Brad

    P.S. I would assume the pressure is atmospheric pressure used in the
    air density calculation. Now whether it is absolute or corrected, I
    couldn't tell you.
     
  5. Guest

    Guest Guest

    Outstanding...Man, that makes perfect sense now...Yes, I had to read it several times /ubbthreads/images/graemlins/wink.gif

    BTW RSi seems to make the adjustment for the other drag tables. BAd thing about RSI is I cant really take it with me. I have a pocket PC and workin off that. Hmmm..Maybe exbal? Right now I'm using "the Thing" as well as Steve Klinkers software Beta. It actually works pretty good

    Joe

    Thanks
    Joe