Ballistic Coefficient

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For many shooters, figuring out the ballistics of their rounds is akin to some arcane form of black magic. There are so many variables involved, and some of these variables have a much greater effect than others. Everyone is basically familiar with the effects of bullet weight and muzzle velocity, and a basic computation of your external ballistics and bullet trajectory can be computed using simple physics formulas that disregard atmospheric conditions and aerodynamic drag using only these components. But there is one aspect of aerodynamic drag that does have a significant measurable effect on bullet trajectory, and that is the ballistic coefficiency of the bullet. But what is a ballistic coefficient?

Hornaday A-Max .308 caliber 208 grain bullet. Note the spire pointed ballistic tip and aerodynamic boat tail, giving this bullet a high ballistic coefficient.

In the simplest of terms, the ballistic coefficient (or BC) of a bullet is the measure of its ability to fly efficiently through the air. Spire points or spitzer rounds obviously pierce through the air better than round nose bullets, and a bullet with a flat base generates much more drag than a bullet with a boat tail design. The number that designates the BC of a round is generally represented as a decimal measured in lb/in², with a higher number indicating a more streamlined bullet with a higher sectional density.

The sectional density of a bullet plays heavily into the resulting ballistic coefficient. The sectional density is the ratio of the diameter of the round and its weight. Computing the sectional density of a bullet is fairly straight forward: simply take the mass of the bullet and divide it by the diameter (caliber) squared. A heavier bullet will have a better (higher) sectional density than a lighter bullet of the same caliber. For this reason, bullets that are lighter tend to have a lower ballistic coefficient than heavier bullets (assuming of course that the bullets have the same aerodynamic shape). Heavier bullets will decelerate less due to the higher inertia their increased weight gives them.

By way of example, a 180 grain round nose soft point .308 Winchester bullet has a BC of around .248, whereas more streamlined and heavier 190 grain spire point boat tail .308 bullets often have a BC exceeding .495. The higher the number, the more streamlined the bullet and the less it will decelerate over time. The lack of deceleration of the bullet gives it a flatter trajectory.

The ballistic coefficient of a bullet doesn't only affect the deceleration of the round and hence it's drop, but it also affects how the bullet responds to cross winds while in flight. Once again we find that more aerodynamic bullets with a higher sectional density are less affected by cross winds when compared to less aerodynamic bullets with a lower sectional density. This explains in part why the common .22 LR cartridge is so affected by bullet drop and cross winds at ranges exceeding 50 yards.

Nosler CT Ballistic Sivertip .308 caliber 150 grain bullet. Note the rounded nose and very short boat tail, giving this bullet a high ballistic coefficient.

For long-range hunters, the ballistic coefficient of a bullet has an enormous effect on the energy a round has when it impacts the target. This is critical for proper performance of the round. For example, consider two Remington Core-Lokt 180 grain .308 bullets fired with a muzzle velocity of 2620 FPS and a muzzle energy of 2743 lb/ft. One bullet is a round nose soft point with a BC of .248. The other is a spire point with a BC of .383. The weight and sectional density of both rounds is the same. Out to 200 yards, we don't see much difference in the performance of these two rounds, but at a range of only 300 yards we begin to see huge differences in their performances; the round nose bullet at this range only has a velocity of 1665 FPS, while the spire point bullet is still traveling at 1974 FPS. At 500 yards, the difference is even more pronounced with the velocity of the round point dropping to 1212 FPS while the spire point is still humming along at 1604 FPS. If these rounds were fired at an elk at a distance of 500 yards, the round nose would hit with only 587 lb/ft of energy, while the faster spitzer bullet would impact with a much greater 1028 lb/ft.

Do you need to worry about the ballistic coefficient of your round? Probably not. For the average shooter, the ballistic coefficient of a given round simply will not have a huge effect on them. For hunters targeting game between 50 and 250 yards, and target shooters plinking at similar ranges, the ballistic coefficient doesn't have much time to affect the flight of the bullet. However, for long range and competition shooters, the importance of having a heavy streamlined bullet is of critical importance as they engage targets at 300 yards and beyond. The aerodynamic deceleration of a round increases exponentially with distance, so a bullet with a low BC will be affected much more at 600 yards when compared to a bullet with a high BC. The ability of a high BC round to overcome wind resistance as well be less affected by cross winds is critical to long range high-power competitors or hunters who hunt game at long ranges.