I was speaking with a few guys at my shooting range about why some bullets have a straighter trajectory than others. One of them, who sounded like he knew what he was talking about, said that if you know the sectional density of a bullet you can calculate its trajectory. This would be useful information, but how do I get the sectional density of the bullets I hunt with? Is there a table of sectional densities for different bullets, or is there a simple formula for calculating sectional density? Dennis Ryburg, Mt. Carmel, PA
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I was speaking with a few guys at my shooting range about why some bullets have a straighter trajectory than others. One of them
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"Sectional density" is a ballistic term much used, abused and confused, as it was by your local "expert." My guess is that he had sectional density confused with "ballistic coefficient," which frequently happens in clubhouse discussions.
In simple terms, a bullet's sectional density (SD) is a numeric expression of its weighttodiameter ratio: Bullets that are heavy have a higher SD than lighter bullets of the same caliber. For example, a 110grain .30 caliber bullet has an SD of .166, whereas a 220grain bullet of the same diameter has an SD twice as high. Though SD can be calculated, it usually isn't worth the bother. Simply looking at two bullets you see which has the higher SD, and it's of little help in the more complex calculations of trajectory. All things being equalwhich they almost never are in the ballistics universebullets with a high SD maintain their momentum better and penetrate deeper than lighter bullets of the same caliber. But don't take this to the bank, because when such variables as velocity, bullet shape and construction are tossed into the stew, terminal performances can vary widely.
As for calculating trajectories, one of these days I'll get around to writing a column about the mysterious "ballistic coefficient," which will provide some understanding as to how trajectory charts are calculated.
Jim Carmichel, Shooting Editor

sectional densities and ballistic coefficientcy have already been charted to trajectory tables that can be found on a manufactures table for that bullet and caliber.the design and type of bullets used on different types of game makes interesting talk around campfires , . the main reason some bullets have flatter trajectory than others is lighter bullets generally shoot faster but have less stored energy .
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I look more at Ballistic Coefficient when calculating trajectory. Many things affect accuracy such as rate of rifling twists, etc. But, I think a bullet with uniform weight distribution around the spin axis will ultimately be the most accurate, all other things being equal...
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Originally posted by outdoorlifeeditor View Post"Sectional density" is a ballistic term much used, abused and confused, as it was by your local "expert." My guess is that he had sectional density confused with "ballistic coefficient," which frequently happens in clubhouse discussions.
In simple terms, a bullet's sectional density (SD) is a numeric expression of its weighttodiameter ratio: Bullets that are heavy have a higher SD than lighter bullets of the same caliber. For example, a 110grain .30 caliber bullet has an SD of .166, whereas a 220grain bullet of the same diameter has an SD twice as high. Though SD can be calculated, it usually isn't worth the bother. Simply looking at two bullets you see which has the higher SD, and it's of little help in the more complex calculations of trajectory. All things being equalwhich they almost never are in the ballistics universebullets with a high SD maintain their momentum better and penetrate deeper than lighter bullets of the same caliber. But don't take this to the bank, because when such variables as velocity, bullet shape and construction are tossed into the stew, terminal performances can vary widely.
As for calculating trajectories, one of these days I'll get around to writing a column about the mysterious "ballistic coefficient," which will provide some understanding as to how trajectory charts are calculated.
Jim Carmichel, Shooting Editor
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