The ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. It is inversly proportional to the decelaration. A high number indicates a low deceleration. BC is a function of mass, diameter, and drag coefficient. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant i that relates to the aerodynamics of its shape. Ballistic coefficient has units of lb/in² or kg/m². Normally BCs are stated in lb/in² by gun projectiles producers without referring to this unit. (Definition by Wikipedia)
Some interesting facts about ballistic coefficient of bullets:
1) Ballistic coefficients are not absolute. They change with temperature, relative humidity, atmospheric pressure and altitude.
2) Ballistic coefficient defines the bullet's relative efficiency.
3) Ballistic coefficient is expressed as a ratio of the drag of a bullet to that of a standard projectile. This number is almost always a decimal number less than 1.000.
4) Bullet manufacturers calculate the ballistic coefficient not only with reference to a standard projectile, but also to standard test conditions. These conditions are altitude (sea level), temperature (59 deg F), atmospheric pressure (29.53 inches mercury) and relative humidity (78%). If conditions are different from these, corrections to the ballistic coefficient must be made.
Basically, the higher the ballistic coefficient, the flatter shooting a bullet of a given diameter will be at a given muzzle velocity; for instance:
Hornady .308 diameter 180 gr Spire Point has BC= 0.425
Hornady .308 diameter 180 gr Round Nose has BC= 0.241
If fired at 3000 feet per second muzzle velocity at standard conditions at various distances the bullet drop will be as follows if zeroed at 100 yards:
100 yds 200 yds 300 yds 400 yds
30 cal Round Nose 0 inch -3.6 inches -14.1 inches -34.0 inches
30 cal Spire Point 0 inch -3.0 inches -11.3 inches -25.9 inches
As you can see, the lower drag pointed style bullet has less drag and drops less over range than the less streamline round nose bullet.