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RCmodel, Walkalong, or another expert please explain S.D.

Discussion in 'Handloading and Reloading' started by gamestalker, Dec 14, 2011.

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  1. gamestalker

    gamestalker member

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    I thought it had something to do with the length of the bearing surface and rate of twist but have no real idea how it computes.

    Having reloaded for over 3 decades one would think I have a complete understanding, I don't and need some information about sectional density please. I have a bunch of reloading manuals, but I have never been able to find out what S.D. is and how, or if, it impacts bullet performance. Does it have an effect on a hunting round regarding bullet integrity such as penetration and, or, expansion.

    I haven't had any problems all these years building loads that produce tight groups, but S.D. is something I've never acknowledged or researched as to how it effects reloading options.
     
  2. 1911Tuner

    1911Tuner Moderator Emeritus

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    I'll take a shot at it.

    Sectional density is the ratio of a bullet's mass to its cross-section.

    All else assumed to be equal, a 150-grain 7mm bullet has greater sectional density than a 150-grain .30 caliber bullet and will have a slightly flatter trajectory at a given muzzle velocity...and it will penetrate a little deeper in a given medium.

    Likewise, a 180 grain .30 caliber bullet has a greater sectional density than a 165-grain .30-caliber bullet.

    The actual difference in performance between the two bullets will be small.
     
  3. Haxby

    Haxby Member

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    I'm no expert, but the experts at Sierra say:

    Sectional Density: A bullets weight, in pounds, divided by its diameter in inches squared. High sectional density is essential to producing a good ballistic coefficient and deep penetration.
     
  4. Red Cent

    Red Cent Member

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    Read about the 6.5 MM bullet. :)
     
  5. Walkalong

    Walkalong Moderator

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    Yep.

    The longer a similar bullet is in the same caliber, the higher the sectional density, and the better the penetration potential, assuming similar construction and expansion.
     
  6. Red Cent

    Red Cent Member

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  7. dmazur

    dmazur Member

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  8. rcmodel

    rcmodel Member in memoriam

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    Explaned another way with a question.

    Say you have a 250 grain 50 cal round lead ball.
    And a 250 grain 28" long, 1/4" dia arrow.

    If both hit a target going 350 FPS, which one would you expect to penetrate fruther?

    The arrow will shoot through a bucket of sand because it has a very high SD.
    The larger dia round ball, not so much.

    rc
     
  9. Mxracer239y

    Mxracer239y Member

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    The experts at Sierra need to review their dimensions :neener:
     
  10. dmazur

    dmazur Member

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    Here's some more on sectional density -

    http://www.gsgroup.co.za/sdplywood.html

    An excerpt, after all the charts, says

    Based on the last table, a direct comparison of penetration and sectional density in the chart above, shows that any link between the two is imaginary, as are comparisons of penetration to speed or momentum or energy. Terminal performance is a highly complex subject and an interaction of a multitude of factors. The closest one could probably come to a single factor for gauging terminal performance, is Momentum/Cross Sectional Area (Mo/XSA) and then only if the numbers are tempered with bullet shape, bullet construction and the effect of speed induced stagnation pressure.

    The bottom line is that, choosing between two bullets based purely on sectional density, is as foolish as choosing a bullet based on the colour of the packaging.
     
  11. Haxby

    Haxby Member

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    Mxracer239y -
    You can look up the formula anywhere.
     
  12. sugarmaker

    sugarmaker Member

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    SD = (weight in grains) / (7000*(d)^2)

    d=diameter in inches

    ex: 400 grain .50 cal

    SD = 400/(7,000 * (.50)^2
    =400/(7,000 * .25)
    =400/1750
    =.229
     
    Last edited: Dec 15, 2011
  13. gamestalker

    gamestalker member

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    I forgot that my #10 Speer manual has just about every deffinition in the back pages of the book, but I'm still very grateful for the comparisons and explanations provided by you guys, as it has really helped me in understanding how SD effectively fits into my reloading and shooting needs.

    Good stuff guys, and thanks again!
    Merry Christmas,
    Gamestalker
     
  14. Jon_Snow

    Jon_Snow Member

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    Just keep in mind that ballistic and terminal performance are going to be much more heavily influenced by other factors like bullet construction and shape.
     
  15. ranger335v

    ranger335v Member

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    "... the experts at Sierra say: Sectional Density: A bullets weight, in pounds, divided by its diameter in inches squared. High sectional density is essential to producing a good ballistic coefficient and deep penetration."

    Well, they are experts but that's misleading, at best. High sectional density, of itself, means virtually nothing. A 180 gr. .30 cal bullet has a SD determined by those numbers, and it will be the same if it's loaded forward or backward but the BC and penatation at 200 yards will be much different.

    All bullets of the same weight and diameter have indentical SD. The bullet's terminal effect is MUCH MORE dependant on construction than anything else! A fragile Nosler BT 150 gr. .30 cal has exactly the same SD as a 150 gr. .30 cal Barnes but the penatration will be much different. I've ignored SD as a meaningful factor in the choice of hunting bullets for about 40 years now, all I care about is the BC and construction for what I want to hit; if I want deeper penatration I'll use a bonded core, a partition or a monolithic type bullet.
     
  16. 1911Tuner

    1911Tuner Moderator Emeritus

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    I think it's safe to assume that they mean bullets loaded nose forward.

    It's also probably safe to assume that the comparison is made with the standard "All else assumed to be equal" disclaimer.
     
  17. Mxracer239y

    Mxracer239y Member

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    :banghead:

    You can. What you will find is that you divide weight by diameter squared (in dimensions of length squared). You do not divide by diameter, as quoted (which would have dimensions of length).

    The quote mentioned diameter having dimensions of length squared. It is a confusion between the 'diameter in inches squared' and 'diameter squared.'
     
    Last edited: Dec 16, 2011
  18. dmazur

    dmazur Member

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    I found the "lead" article, before the plywood tests on penetration -

    http://www.gsgroup.co.za/articlesd.html

    This one is titled "Sectional Density - A Practical Joke?" and I think it is worth reading.

    The conclusion seems to be that yes, one can calculate sectional density, but it doesn't really appear to have any practical application. Nevertheless, the term is still tossed around as if it did.

    There are better measures of bullet performance, but shooters seem to want a single, all-inclusive term.

    I don't think sectional density is the answer.
     
  19. 1911Tuner

    1911Tuner Moderator Emeritus

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    It's not. It's basically just a piece of information that means little by itself.
     
  20. ranger335v

    ranger335v Member

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    "A 180 gr. .30 cal bullet has a SD determined by those numbers, and it will be the same if it's loaded forward or backward but the BC and penatation at 200 yards will be much different."

    "I think it's safe to assume that they mean bullets loaded nose forward."

    So we would assume but what "they mean" wasn't the point; loaded either direction doesn't change the SD and that IS the point!

    My example simply proves that bullet weight, per se, provides no automatic benefit to the BC. (Boat tails generally add to BC but not so much on backward spitzers, which would effectively make them boat tail wad-cutters!)


    "All bullets of the same weight and diameter have indentical SD. The bullet's terminal effect is MUCH MORE dependant on construction than anything else!"

    "It's also probably safe to assume that the comparison is made with the standard "All else assumed to be equal" disclaimer."

    Yeah. But it's also safe to assume that all things are rarely "equal." I'm talking real world bullets here, not isolated ideas in some anal theory.
     
  21. Haxby

    Haxby Member

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    Chuck Hawks - To calculate a bullet's sectional density divide the bullet's weight (in pounds) by its diameter (in inches), squared.

    Merriam - Webster Dictionary - Definition of SECTIONAL DENSITY
    : the ratio of the weight of a projectile to the square of its diameter

    (Lilja) riflebarrels.com - sectional density is defined: SD=w/d^2

    Hornady - Sectional density (a bullet’s weight in pounds divided by its diameter squared)
    Nosler - The ratio of a bullet's weight, in pounds, to the square of its diameter, in inches.

    Speer - A bullet's weight in pounds divided by the square of the diameter in inches.

    There is no mention of area.
     
  22. denton

    denton Member

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    There is no mention of area because the area of a circle is directly proportional to its diameter squared. Since SD is just a comparative number, there is no point in also applying the pi/4 terms. Remember, this was invented back when there were no four function calculators.

    SD does influence BC. Long skinny bullets have higher BC and higher SD.

    The other use for SD is to answer the question, if I like an X grain bullet in my Y rifle, what weight bullet gives the same proportions in the Z rifle? So, IIRC, if you scale a 180 grain 30 cal bullet down to 7mm, you get something like 162 grains.
     
  23. ArchAngelCD

    ArchAngelCD Member

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    The picture drawn here with words makes SD very clear IMO.
     
  24. dmazur

    dmazur Member

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    Yes, it is a clear picture. I'm not sure it is at all related to bullet performance, however.

    I'm beginning to believe that SD is only part of the story, and discussing it as an isolated entity is misleading at best.
     
  25. 1911Tuner

    1911Tuner Moderator Emeritus

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    We understand that. The OP's request was: "Please define Sectional Density." So defined, all else must be assumed to be theoretically equal in order to satisfactorily explain it. That being: Assuming equal construction...range...initial velocity...impact velocity...atmospheric conditions...and target medium, the bullet with higher sectional density will shoot a little flatter and penetrate a little deeper.

    "Anal" theories over bullet performance in the real world pretty much fall under another topic. We can add that actual bullet performance depends more on construction...range...initial velocity...impact velocity...atmospheric conditions..and target medium without going into firing into ballistic gel vs a living animal or silliness such as loading the bullet backward which nobody in the real world is going to do.

    :)
     
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