Ballistic coefficient, usually shortened to BC, is a number that describes how well a bullet retains velocity as it travels through the air.
A bullet with a higher BC generally slows down more slowly, which means it retains velocity better and is affected less by wind over longer distances.
BC is determined mainly by two things: the bullet’s weight relative to its diameter, and how aerodynamic its shape is. A heavier, more streamlined bullet will generally have a higher BC than a lighter or less aerodynamic one. It’s helpful to think about this in terms of bullet shape. A long, tapered nose with a long boattail will generally have a higher BC than a round-nose bullet with a flat base. Those are opposite ends of the spectrum, but they make the concept easy to visualize.
You’ll commonly see BC listed as either G1 or G7. These are simply different reference standards used to describe how the bullet flies. Modern long, pointed, boattail bullets are usually better represented by a G7 BC.
For most shooters, the important thing to understand is simple: a higher BC means the bullet retains velocity better in flight.
If you want to understand how BC is calculated, why G1 and G7 numbers are different, and how BC affects your ballistic solution, read our full article: What Is Ballistic Coefficient? A Simple Explanation.