There are different coefficients that can be associated with a bullet, but the most common one is the ballistic coefficient (BC).  The ballistic coefficient of a bullet is a measure of its ability to overcome air resistance as it travels through the atmosphere. It is determined by comparing the bullet's flight characteristics to those of a standard projectile, such as a sphere or a G1 or G7 model.  A high ballistic coefficient indicates that a bullet has a relatively low drag and will maintain its velocity and energy better over long distances. A low ballistic coefficient means a bullet will slow down more quickly and may be less accurate at longer ranges.  The ballistic coefficient is an important factor to consider when selecting ammunition for long-range shooting or hunting.