Ballistic Coefficient - Bullet Performance

Bullet Performance

The formula for calculating the ballistic coefficient for bullets only is as follows:

where:

• BC_Bullets = ballistic coefficient
• SD = sectional density, SD = mass of bullet in pounds or kilograms divided by its caliber squared in inches or meters; units are lb/in2 or kg/m2.
• i = form factor, i = ; (C_G ~ 0.5191)
• C_B = Drag coefficient of the bullet
• C_G = Drag coefficient of the G1 model bullet
• M = Mass of bullet, lb or kg
• d = diameter of bullet, in or m

This BC formula gives the ratio of ballistic efficiency compared to the standard G1 model projectile. The standard G1 projectile originates from the "C" standard reference projectile (a 1 pound (454 g), 1 inch (25.4 mm) diameter projectile with a flat base, a length of 3 inches (76.2 mm), and a 2 inch (50.8 mm) radius tangential curve for the point) defined by the German steel, ammunition and armaments manufacturer Krupp in 1881. By definition, the G1 model standard projectile has a BC of 1. The French Gavre Commission decided to use this projectile as their first reference projectile, giving the G1 name.

A bullet with a high BC will travel farther than one with a low BC because it is affected less by air resistance, and retains more of its initial velocity as it flies downrange from the muzzle (see external ballistics).

When hunting with a rifle, a higher BC is desirable for several reasons. A higher BC results in a flatter trajectory for a given distance, which in turn reduces the effect of errors in estimating the distance to the target. This is particularly important when attempting a clean hit on the vital organs of a game animal. If the target animal is closer than estimated, then the bullet will hit higher than expected. Conversely, if the animal is further than estimated the bullet will hit lower than expected. Such a difference from the point of aim can often make the difference between a clean kill and a wounded animal.

This difference in trajectories becomes more critical at longer ranges. For some cartridges, the difference in two bullet designs fired from the same rifle can result in a difference between the two of over 30 cm (1 foot) at 500 meters (550 yards). The difference in impact energy can also be great because kinetic energy depends on the square of the velocity. A bullet with a high BC arrives at the target faster and with more energy than one with a low BC.

Since the higher BC bullet gets to the target faster, there is also less time for it to be affected by any crosswind.