I’m sure everyone here knows that a bullet is most effective when it’s traveling through the air at its prescribed velocity. After all, that’s what the bullet is designed to do. The cartridge holds the powder, and once the powder ignites, initiated by the primer, the projectile unseats from the casing, travels the length of the barrel, and goes about its merry way. But let’s not forget the final factor to consider before the projectile strikes home—its “happy place,” the air.
Over the course of the projectile’s short airborne life, what are its ideal living conditions? Here is a short breakdown of what a projectile likes as it exits the end of your rifle barrel. This brief analysis of the significance of ambient air density will hopefully give you a better understanding of environmentals that you’ll be able to apply every time you hit the range. Be sure to track the data over time and look at the differences from engagement to engagement, analyzing the weather.
Bullet Ambient air density
Ambient air density is classified into four categories: air temperature, barometric pressure, humidity, and density altitude.
Air temperature: As the air temperature rises, the air density is lowered. Because there is less resistance, the velocity of the round will increase, causing the point of impact to rise. Note that this is in relation to the air temperature at which the rifle was zeroed. If you were to zero your rifle when the air temperature was 60 degrees, but fire the rifle in a temperature of 100 degrees, the point of impact will rise considerably.
Barometric pressure: Barometric pressure is often referred to as atmospheric pressure, and is the force that is exerted on objects by the weight of the atmosphere above them. Though we may think of gas as not weighing on anything, it does, in fact, have mass. Because of this and the effect of gravity upon the gas itself, the air above us and around us does weigh down on us, and will in the same manner on a bullet.
The barometric pressure is measured in the downward force that the atmosphere exerts per unit of measure in a certain given area. In the realm of precision shooting, the air pressure is less at higher altitudes and the air is less dense. This means that the bullet is more efficient due to having less drag.
Look at it in terms of airliners. When jumbo jets take off and reach an altitude of 30,000 feet above ground level where the air is less dense, they can average speeds of 580 mph. The same jumbo jet would have a hard time achieving this speed at 20 feet above ground level, simply due to the fact that the air is more dense there, thus creating more drag.
Humidity: Humidity has a counterintuitive impact (Not what FM 23-10 tells you). Since water vapor has a density of 0.8 grams per liter while dry air averages approximately 1.225 grams per liter, the higher humidity decreases the air density, thus decreasing the drag on the bullet.
Density altitude: Density altitude is perhaps the single most important factor affecting the bullet’s performance when shooting at extended ranges. Density altitude is the pressure altitude adjusted for non-standard temperature. Both an increase in the temperature and, to a much lesser degree, humidity will cause an increase in the density altitude. This means that in hot and humid conditions, the density altitude at a certain location may be significantly higher than the true altitude.
This is important when shooting a precision rifle. We know that in higher altitudes, the bullet will experience less atmospheric drag, thus giving it the ability to fly faster and farther. A high density altitude will do the same, even if your true altitude measures otherwise. In other words, the bullet doesn’t care about true altitude; it will perform according to what it “feels” as it flies through the air in density altitude.
Featured photo courtesy of forensicoutreach.com