Sectional Density Importance

Discussion in 'Firearms & Ammunition' started by Andy Spencer, Jun 27, 2018.

  1. Andy Spencer

    Andy Spencer New Member

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    How important would you rate this factor before buying a new gun for Africa?

    Honestly, I just discovered it and want to make a good decision using this as one more piece of the puzzle. Just how much does it matter?
     

  2. CTDolan

    CTDolan AH Elite

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    It matters but not as much as most think.

    As a rule, if you're looking into a rifle for Africa, for dangerous game in particular, go with a proven cartridge in a proven rifle. My recommendation? A .375 H&H in a decent bolt action (M98, M70, or similar). Want something bigger? Provided you can manage the recoil, the .404 Jeffrey is making a strong comeback. The .416 Rigby has been selling well for the last 20 years (it was never all that popular when Africa was in her prime but sells very well today). Want something more modern? Go for a .416 Remington Magnum. Get one in a new Winchester M70. Everybody that has one seems to love it, big time. Not afraid of dropping some serious cash? Well then, you are blessed indeed and the world is your oyster!

    When it comes to sectional density the .458 class of cartridges are wonderful. But, it's a lot of gun for the uninitiated (and really unnecessary for a client on a safari).
     

  3. curtism1234

    curtism1234 AH Enthusiast

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    Imo the new premium tough bullets on the market today kind of makes this an outdated argument unless we are talking about cheap cup and core ammo.
     

  4. Ray B

    Ray B AH Elite

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    SD is only the average length of the bullet, without regard for caliber or bullet shape. I know this sounds incorrect because SD has been used as the holy grail for estimating a bullets ability to penetrate and other characteristics. If you disagree, just look at the calculation: bullet weight divided by quantity diameter x diameter. Since bullets are round and diameter x diameter would give the area for a square bullet, the SD function gives a proportionate number to the other option: bullet weight divided by bullet area (cross-section). So if bullets were cylindrical and had the same density, then all bullets of all calibers with the same SD would be the same length. A .224" bullet with an SD of .250 would be the same length as a .308" as a .458". Although they would be the same length, their shapes would be entirely different. The .224" bullet will be very long relative to its diameter, while the .458" will be very short relative to its diameter.

    In terms of penetration, the bullet is slowed by stuff getting in the bullets way which is to say that the .458" bullet has more surface area to contact more stuff than the .224" bullet. If bullets were non-expanding so the formula was kept simple, the smaller diameter bullet would penetrate farther than the larger diameter.

    So the solution to penetration (keeping strictly to the shape issue, bullet construction is separate) is the ratio of bullet weight to diameter. Based on the experiences of several hunters and their printed comments regarding what bullets worked well for them, it appears that a ratio of what would be termed "three calibers" is a good starting point. That is to say, the bullet will average three times longer than its diameter. The actual bullet will be slightly longer than the three calibers because the rounded point extends its average length. To achieve the three calibers measure, as the bullet diameter is increased, the SD will also noted to increase. For example, a .375" bullet with 3 calibers will have a much smaller SD than a .458" bullet but there are examples of bullets that will have similar penetration.

    That's a long way of saying, SD is only a guideline when seeking penetration potential. Bullet weight and frontal area as well as construction, velocity and spin are also factors to consider.
     
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  5. ChrisG

    ChrisG BRONZE SUPPORTER AH Fanatic

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    I have to ask.... why is your avatar a bust of Nero? Isn't that a bit like having an avatar of John Wayne Gacy?
     

  6. Ridgewalker

    Ridgewalker AH ENABLER LIFETIME BRONZE BENEFACTOR AH Legend

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    Andy, IME it was more about what I could shoot well multiple times in a legal DG caliber than anything else. That is to say I fired a buddy’s 416 Rigby off sticks and it took me so long to get back into battery for a follow up that I backed off to a 375 H&H. Even with it (nearly 10 lbs loaded) I added a muzzle brake primarily for DG hunting and now I can accurately fire nearly 3 rounds by the time my buddy is working on his second round. I know some PHs don’t like muzzle brakes and I understand that, but if I’m facing DG I’m going to be prepared to do the best I can for my safety and the animal’s humane killing.
    JMO derived from my minimal experience.
    Best of luck in your decision!
     
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  7. Andy Spencer

    Andy Spencer New Member

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    Turns out I look like him. Cousins sent me a picture from there vacation. Just said you look like ceasar. Didnt know it was a bust of nero the tyrant. So yea, sorta weird but accidental.
     
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  8. flatwater bill

    flatwater bill AH Elite

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    RayB............Nice post on sectional density. I have talked with many experienced, even famous, hunters who did not truly understand what it is. You gave a concise and correct definition........thanks...................FWB
     

  9. 375 Ruger Fan

    375 Ruger Fan AH ENABLER GOLD SUPPORTER AH Legend

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    Last edited: Jun 30, 2018
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  10. Ray B

    Ray B AH Elite

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    I realize that Chuck Hawks gets paid well to express his opinions, but his explanation of SD contains too many un-noted variables and other causes to give validity to his conclusions on the effects of SD. See sentence two of my previous post.
     

  11. PHOENIX PHIL

    PHOENIX PHIL AH ENABLER AH Ambassador

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    There's a lot more explanation than necessary there. For a given caliber, there's only one way to improve SD and that's to add weight.

    More weight behind something with the same frontal diameter, means more penetration. The explanation could have stopped at the comparison of the 1/4" wooden dowel versus the 1/4" steel rod.

    If you want to take the explanation further and actually use a physics term, then talk about momentum. Momentum has the parameter of direction. It is momentum we want to optimize when seeking deeper penetration with a bullet.
     

  12. 7x57Joe

    7x57Joe AH Veteran

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    That's why depleted uranium works so well from a GAU-8/A 30mm against armor plate.
     
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  13. Ray B

    Ray B AH Elite

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    The confusion seems to come from a mis-named characteristic. Sectional Density should mean how dense is the bullet per unit of cross-section. In use it only means how much weight there is per unit of cross-section. No consideration is given to the shape and only indirectly to the material density. There would be less confusion if (weight x diameter x diameter) was called Sectional Weight and the definition of Sectional Density involve bullet bore-length ratio which would include actual density.
     

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