We all know that temperature can affect a shooter in a number of ways; from uncomfortably excessive heat waves to ice-cold wind chills. But what often baffles the novice shooter is how much of an effect temperature has on a bullet during flight. Many in fact aren’t even aware of the effects, and those that are often concern themselves over the wrong data. My aim today is to clearly iron out this topic and provide some guidance on how to compensate for temperature effects on your bullet, so without further-ado, the question of the day – how does temperature affect a bullet during flight?
Temperature alters a bullet’s trajectory in two ways. The first change takes place in the chamber and cartridge itself, known as the internal temperature, while the second is of the air temperature outside, known as the external temperature. As the temperature rises, muzzle velocity increases while air density reduces, producing less drag onto the bullet. This flattens the trajectory, resulting in high hits if compensation is not made.
A temperature increase of only 5°C on the day of shooting is capable of displacing your bullet by up to 20cm (or 8-inches) between shots, while complete seasonal changes may have far more drastic effects on a rifle’s DOPE charts. For this reason, it is vitally important that the long range shooter monitors temperature, and compensates for it accordingly.
So How Does Temperature Affect a Bullet?
There are two factors to consider when determining and compensating for temperature effects on your bullet. These are the internal and external temperatures.
This is the temperature of the cartridge and propellant itself. As the rifle cartridge heats with rising ambient temperature, the propellant’s burn rate is altered, causing indifferent chamber pressures. This increase in internal temperature will cause the nitrocellulose based powder (modern gun powder) inside the cartridge to burn at a higher rate, causing an increase in pressure and velocity, and therefore a flatter trajectory, impacting high.
So to put this into simpler terms – as the gunpowder warms in hotter temperatures, it creates a faster muzzle velocity and flattens out the bullet’s trajectory.
Certain rifle powders are more affected by temperature change than others. These are known as temperature-sensitive powders. Despite what some manufacturers might claim, all powders are affected by temperature, however some are not as severe as others.
Some powders may experience a burning-rate increase of 3.5fps (or 1mps) for every 1°C increase in ambient temperature, while others will be more resistant to heat and may only have an increase of 1.5 fps (or 0.45mps) for every 1°C increase.
This is the actual outside air temperature, otherwise known as the ambient temperature.
- Cold Air – a cooler air temperature will always result in lower impacts on target, as cold air is denser (thicker and heavier) than warm air, thus producing more drag on the bullet.
- Warm Air – if the air temperature increases, your bullet’s trajectory will flatten out due to a lower air density (less collisions with air particles). The result will be a high impact of the bullet if this effect is not accounted for.
High temperatures not only flatten a bullet’s trajectory, but also introduce mirage. Mirage is a good indication of wind speed and direction at the target end, but can also cause image distortion whereby the target appears to be moving in a wavy motion. In this case, reduce scope magnification for a clearer image.
How Much Effect Does Temperature Have on my Bullet?
While the external temperature effects will remain fairly consistent from one shooter to the next, the internal effects are largely determined by the propellant which is being used, and how sensitive that propellant is to temperature change.
The following chart will demonstrate how much of an effect temperature may have upon a projectile during flight when fired in various environments. This specific data comes from a 168gr Hornady HPBT Match bullet, and a rifle propellant that causes a 2.5fps change in muzzle velocity for every 1°C change in ambient temperature.
We can clearly see by looking at the table above, that as temperature climbs and air density reduces, the bullet’s trajectory gets flatter. It is also evident that the rifle’s muzzle velocity increases in line with temperature. The above bullet shot from the same rifle may in this case experience a bullet displacement of over 1 meter when fired over a 900 meter distance and with a 20°C change in air temperature.
We can also notice that at 300 meters, the difference in bullet drop is almost negligible; a couple of centimetres at most. This is why seasoned hunters may not even be aware of this temperature influence; because they do not need to account for it to hit their target. But for long range precision shooting, it is vital.
How to Compensate for the Effects of Temperature
Now that we understand how temperature can alter trajectory, we need to known how to correct this effect to ensure pinpoint accuracy regardless of the weather conditions.
While many military snipers and sharpshooters are often taught to use a 1MOA shift for every 20ºC change in ambient temperature, it would be far more accurate to figure out the actual effect of temperature on your own rifle and cartridge combination. The process is fairly simple and easy.
- Internal Temperature Compensation – to determine what effect temperature will have on your muzzle velocity, you need to figure out how sensitive your propellant is to temperature change. This is achieved by firing your rifle through a chronograph and recording the muzzle velocity over various temperatures. Simple math will then allow you to determine what a 5 or 10 degree change will do to your muzzle velocity. When using ballistics software to produce your trajectory or clicks required to engage various targets, enter the muzzle velocity of your rifle as it would be under the current temperature.
- External Temperature Compensation – this one is easy. Simply enter the current atmospherics (temperature, barometric pressure and relative humidity) into your ballistic calculator, and it will do the rest for you.
Tip: some ballistic app’s such as Strelok Pro provide options for you to enter muzzle velocity recordings at various temperatures while you shoot. These app’s determine your propellant’s temperature sensitivity for you and automatically update your muzzle velocity in line with the current air temperature.
While this may all seem fairly complicated, it becomes simpler once put into practice. It is very common for many shooters to determine a rifle’s muzzle velocity once through ammo testing or load development, and then stick to that velocity reading regardless of the outside conditions. We can now see however that at longer distances this will cause a large displacement of the bulled in relation to your point of aim, so temperature needs to be monitored and accounted for at regular intervals while shooting.
To quickly recap what we have covered above:
- Muzzle velocity and propellant sensitivity need to be determined and updated as you shoot; in order to provide the most accurate trajectory possible.
- Ambient air temperature needs to be entered into ballistics software and updated throughout the day as and when required.
If you are still a little confused and need a further explanation in compensating for the effects of temperature on bullet flight, please leave a comment below. And if you found this article helpful and would like to learn more about atmospheric effects on a projectile, I’d recommend reading the following write-up: The Effects of Altitude and Barometric Pressure on a Bullet During Flight.