Sectional Density and Ballistic Coefficient.... Can some school me on this?

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  • Brandon_SPC

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    So with my new AR-15 build being a 6.5 Grendel and just got into reloading, I have been reading on sectional density and ballistic coefficient. I have never really been curious about it until now because I have never really had the need. From my understanding if I am correct BC is related to the bullet ability to overcome air resistance the higher the BC the better or from what I have read the "slicker" the bullet. But the sectional density is what I am having trouble comprehending. When does this matter? When does it matter to apply towards game such as coyotes, deer, elk, etc? (speaking in general not this particular cartridge) I eventually want to start poking out to 400+ yards and and experiment with shooting long range...
     

    Brandon_SPC

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    I just realized I might have posted this in the wrong section. I didn't see the Long Range section. My apologies.
     

    boatbum101

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    You got BC pretty much dead on . Sectional density relates to a bullets ability to penetrate . The higher the SD the more a bullet will penetrate . For thin skinned lightly built ( ie coyote , small deer ) you'd want a lower sectional density , for heavy built & bone ( ie elk , moose , bear ) you'd want higher sectional density . Hogs unless 300 + somewhere in the middle will work . For instance take a 6.5mm 140gr bullet long & thin like a pencil it'll have the same sectional density as a .308 200 grain bullet . That why those crazy Swedes use the 6.5X55 on moose & even polar bears . Those long thin bullets penetrate like no tomorrow .
     

    bigbulls

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    Sectional density is nothing more than a bullets weight in relation to the diameter of the bullet.

    The more the bullet weighs per a given diameter the higher the sectional density and the better it will penetrate, assuming the same type of bullet.
     

    SAWMAN

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    Yep,what Kerry sez. Realistically though,I have never worried about the S.D. of any bullet that I have shot. Even in the long(er) range hunting environment. Killin' moose and bear at <50yds with my 45-70 using a 405gr WFNGC,or shooting yotes waaaaay out there with my .30-378 Wby Mag and a 220gr SMK I have always not felt the "need" to worry about the S.D. thing.

    And of course,anything punching paper,the S.D. of your bullet is really not a factor,IMO. I think that concentrating on just how accurate you can make your load come outta your gun is much more important. Is for me anyway. --- SAWMAN
     

    M118LR

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    Well, perhaps SD is part of the BC equation. But yes, BC is of greater concern when flight times increase and SD is of more concequencience when penetration is a concern.


    This formula is for calculating the ballistic coefficient within the small arms shooting community, but is redundant with BCProjectile:

    {\displaystyle BC_{Smallarms}={\frac {SD}{i}}}
    BC_{{Smallarms}}={\frac  {SD}{i}}
    [12]
    Where:

     
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    FrommerStop

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    I have a 6.5 grendel in an AR with 18 inch barrel. It is not a 6.5 Creedmoore. But it is light on recoil and the steel case ammo is supper cheap. The russian ammo If I am lucky is doing about 2550 I figure. I have some barnes 100 grain bullets that I plan to load to about 2700 fps. Should make hits at 250 easy and at about what is my max range need of reaching out to 400 I will have to hold over. For me it is a self defense cartridge and rifle. It would be very hard to legally justify shooting someone in most scenarios at ranges over 100 yards, but a need for longer range could happen.
     
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