By Ryan Cleckner, Gun Digest
In the long-distance ring, temperature plays a much bigger role than simply how comfortable you are while shooting.
How does air temperature affect bullet trajectory?
- Ambient air temperature has an inverse effect on air density.
- This can create a balancing effect in elevation change.
- The change in air temperature can affect bullet drop measured in inches at some ranges.
- Air temperature also effects powders, making them burn hotter and faster.
As a refresher from the last few columns, the reason your bullet falls more by the time it reaches a 900-yard target than it does for a 100-yard target is because it was exposed to gravity longer. Therefore, the longer it takes to reach the target, the more gravity and wind will move the bullet off of its original path.
A Bullet’s Speed
There are three variables that determine a bullet’s speed on its way to the target:
- The initial speed of the bullet
- The efficiency of the bullet
- External/environmental variables
– Air Pressure
In discussing a bullet’s speed, we’ve already covered the initial speed of the bullet, the aerodynamic efficiency of the bullet and the first of the external/environmental variables — air pressure. Now it’s time to explore temperature’s effect.
Each of the three listed variables can change the density of the air. Simply, a bullet doesn’t travel as well through dense, thick air. We know that air pressure is directly related to air density — when one goes up, so does the other.
More air pressure = denser air = slower bullet = lower impact on target
Now it’s time to move on to temperature’s role. Temperature affects a bullet’s speed in two ways: The ambient air temperature plays a role in the air’s pressure, and the temperature of your cartridge — more accurately, the temperature of the powder in your cartridge — will cause varying velocities.
Ambient Air Temperature
Ambient air temperature has an inverse effect on air’s density. As the air’s temperature increases, its density decreases.
This can create a balancing effect of changing altitudes. Typically, when you go higher in altitude, the air pressure decreases (all else being equal). However, the higher you go, the temperature also typically drops. Therefore, increased elevation will likely cause less air pressure (resulting in a faster bullet through less dense air), but it will also cause lower temperatures (resulting in a slower bullet through more dense air).
The point at which these two variables “cancel” each other out is different for each bullet and velocity combination. As we’ll cover in the next column on humidity, there is a universal figure that we can use that takes all three of these environmental variables into account.
Air temperature can actually be the most important environmental variable because it’s so often overlooked. When you’re aware of environmental effects and you zero your rifle at one altitude before going on a once-in-a-lifetime sheep hunt at a different altitude, you aren’t likely to forget to account for the air pressure difference. However, when you’ve been busy and haven’t been able to make it to the shooting range in a while, it’s fairly common to forget to account for the difference in temperature from a few months earlier when you last zeroed/gathered data on your rifle and ammunition.
How much of a difference can this make? Great question. If you’re shooting 175-grain Federal Gold Medal Match bullets out of a .308 Win. rifle at about 2,600 fps on a 55-degree winter day, you can expect about 223 inches of drop from your 100-yard zero at 800 yards. If you didn’t get back to the range until it was 95 degrees in the summer and you expected to make an adjustment on your scope to account for the 223 inches your bullet previously dropped at 800 yards, then you’d miss where you were aiming by about 10 inches.
The other influence on bullet velocity due to temperature has to do with the temperature of the powder in your cartridge of ammunition. This variable is unique because it doesn’t just change with the outside air temperature and weather: It can change even though the environment is exactly the same. If you’re shooting multiple rounds and heat up the chamber of your rifle, and then you let the next round sit in your chamber for a while, you can increase the temperature of your powder.
Hotter temperature creates a hotter and faster-burning powder. This usually results in higher muzzle velocities.
How much of an effect hotter powder has is dependent upon the type of powder you’re using and the bullet/cartridge combination you’re shooting.
Certain powders are more affected by temperature changes. These are called “temperature-sensitive” powders. Now, despite what some manufacturers might claim, all powders are affected by temperature. However, some are not as sensitive as others.
It’s not required that you use a temperature-insensitive powder. Of course, it can help (especially if you don’t want to/don’t know how to account for temperature change). All that’s required is that you track how your rifle and ammunition perform at different temperatures.
You should either invest in a chronograph or make friends with someone who owns one. Then, when you’re shooting in a different temperature, take the time to shoot a couple of rounds through the chronograph to record how sensitive your ammunition is to temperature changes. You should also shoot at distance and note any changes to impact due to the temperature (both due to the different initial bullet velocity and also the different air pressure).
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