Ballistics Basics: Bullet Efficiency And Ballistic Coefficients

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How aerodynamic a bullet is certainly has an appreciable effect on that bullet’s speed on its way to the target.

• The more efficient a bullet is, the better it will maintain its speed.
• And this means it will also be less affected by external variables.
• A bullet's ballistic coefficient is calculated by a model based off its density and shape.
• A bullet's BC will differ depending on whether the G1 or G7 drag model is used.

We have discussed how gravity and wind are the two main variables that affect your bullet’s path. The longer they have to act on your projectile, the more of an effect they’ll have. For example, your bullet doesn’t fall more at farther distances because of the distance (the bullet doesn’t know how far it is traveling), but rather because it takes longer to reach a farther target, and therefore it has more time to fall due to gravity.

There are three variables that determine a bullet’s speed on its way to the target:

1. The initial speed of the bullet
2. The efficiency of the bullet
3. External/environmental variables

We explored the first variable, the initial speed of the bullet, in the last column. Simply, the faster one starts out, the more remaining velocity it will have at the target. Although a faster bullet isn’t necessarily more accurate, it’s affected less by gravity and wind.

Time To Slow Down

All bullets start to slow down the moment they leave the barrel. This is because of drag caused by wind resistance. How much a bullet slows down depends on the density (thickness) of the air and the efficiency of the bullet.

External/environmental variables, such as air pressure, temperature, altitude and humidity can change the density of the air. These variables, which we’ll discuss in the next column, can change each time and location you shoot. Therefore, these variables are mostly a concern when you’re trying to determine the change in your bullet’s speed from the last time(s) you went shooting.

Efficiency Matters

A more efficient bullet will maintain its speed better, and it’s not affected as much by the environmental variables.

Think about it this way: An arrow is more aerodynamically efficient than a tennis ball. Both can be launched at the same speed, but an arrow will fly farther because it doesn’t slow down as much due to wind resistance and therefore maintains its speed better. They are both going to fall at the same rate due to gravity, but the arrow will travel farther in the amount of time it takes both objects to fall.

If we change the density of what the two objects are traveling through, the less-efficient tennis ball will be affected by the change more. As an extreme example, let’s use water because it is easy to picture how much denser it is than air. Both objects will slow down much faster in water, but the arrow won’t slow down as much from its original speed in the air as will the tennis ball. Conversely, if we launched the two objects in a vacuum (no air resistance), then both would continue to fly at the same speed.

These hypothetical examples show us two things: 1) A more aerodynamically efficient bullet won’t slow down as much on its way to the target, and 2) efficiency doesn’t matter as much when the air density is low (thin air).

Bullet Efficiency

A bullet’s efficiency is measured by its ballistic coefficient (BC). The higher the BC, the less drag on the bullet.

The BC of a bullet is a ratio calculated by a mathematical model based off of its density and its particular shape. The best styles for long-range shooting will have high density and aerodynamically efficient shapes. Bullets for hunting, however, might benefit from not having an aerodynamic shape. For example, a lead round-nosed bullet can sure pack a punch! It doesn’t pass through the air, or the animal, easily.

A bullet’s density is a ratio of a bullet’s mass and its cross-sectional area. Simply, large and light bullets are less dense than small and heavy ones. For example, a ping-pong ball is less dense than a marble.

For BC calculations, the shape of a bullet is compared to pre-determined “drag models.” The two most common drag models are G1 and G7. The G1 drag model is based off of a stereotypical spitzer bullet (pointed nose) with a flat base. The G7 drag model, which is gaining popularity, is based off of a more aerodynamically shaped options with a boat-tail base (it tapers back).

A particular bullet will have a different BC depending on whether the G1 or G7 drag model is used. This is because the value for the shape of the bullet is determined by how close its shape is to the drag model. A decent BC for a bullet based on the G1 drag model is in the 0.5-0.6 range, whereas the same bullet’s BC based on the G7 drag model will be in the 0.2-0.3 range.

This difference in the two BCs for the same bullet is because an efficient bullet’s performance will be much better than the G1 model and only slightly better than the G7 model. If a bowling ball was used for the drag model, the BC would be very high because the bullet would be much more efficient as it flew through the air.

Be careful here. Manufacturers might be tempted to advertise high BCs for their bullets, and the comparison to the drag model is more complicated than I made it out to be and actually changes with speed.

Does the actual BC matter? Absolutely not. Instead, the BC is best used as a comparative number between different bullets. Don’t chase a particular BC. Instead, when you’re making the decision on which one to use for the best long-range performance, choose the one that shoots well in your rifle first. If more than one bullet shoots equally well, then choose the one with the higher BC.

This article originally appeared in the Fall 2017 issue of Gun Digest the Magazine.