When a bullet flies through the air, forces called “drag” are exerted upon it. These forces slow down the bullet as it flies, but they don’t act the same way on every bullet. While the subject of aerodynamics is extremely complex, one way to account for differences in bullet drag that is commonly used in the world of small arms is the “ballistic coefficient”.
A ballistic coefficient is a comparative value for a given bullet, showing its relative resistance to drag versus a model projectile that has an empirically established set of drag characteristics. There can be as many model projectiles as there are different bullets, but which ballistic coefficient type is used depends on the model and how good of a match it is for the bullet being measured.
There are nine primary drag models, two of which are commonly used.
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