Temperature and humidity affect bullet trajectory.
Lower temperatures precipitate denser air, causing more bullet drop and more drag. On the flip side, higher temperatures cause thinner air, resulting in less bullet drop and less drag.
These effects explain why bullet manufacturers use the “Standard Metro” to test how well a bullet will perform in the air. The Standard Metros assumes that at sea level, the humidity is 78% while the temperature is 59 degrees Fahrenheit.
Let’s investigate this further.
How Does Temperature Affect Your Shooting?
Temperature changes the bullet path in two ways.
The first change occurs in the chamber and cartridge, referred to as internal temperature. The second change happens outside the chamber, known as external temperature.
As temperature increases, the muzzle velocity increases as the air density reduces, causing less bullet drag. This action flattens the bullet path, resulting in high hits if there’s no compensation.
A temperature increase of 5 degrees Celsius can displace your bullet by as much as 8 inches between shots. Complete seasonal temperature changes, on the other hand, can result in far more drastic effects.
Therefore, you must monitor temperatures before heading to the hunting fields or practice range, especially for long-range shooting.
Internal and External Temperature
Let's expound on the internal and external temperature dynamics to understand better how temperature affects bullet trajectory during flight.
This refers to the temperature generated by the propellant and the cartridge. As your rifle's cartridge heats up, it increases the ambient temperature, changing the propellant's burn rate, which further causes different chamber pressures.
The increased internal temperature burns the gunpowder inside the cartridge at a higher rate, resulting in higher pressure and velocity and, therefore, a flatter trajectory and a higher impact.
Simply, as the gunpowder temperature increases, it generates a faster muzzle velocity, flattening out the bullet's trajectory.
Some rifle powders, known as temperature-sensitive powders, are more susceptible to temperature changes than others.
Temperature-sensitive gun powders may have a burning rate increase of 3.5 frames per second (fps) for every 1-degree Celsius increase in ambient temperature. At the same time, their more heat-resistant counterparts may only experience a 1.5 fps for every 1-degree Celsius increase.
External temperature is broken into two categories:
Cool Air: Cool air temperature results in low impact on your target because colder air is thicker and heavier than warm air, generating more drag on your bullet.
Warm Air: An increase in air temperature flattens your bullet’s trajectory because the bullet runs into fewer air particles, leading to a high impact on your target.
Side Note: High temperatures do more than flatten a bullet path. It also introduces mirage. For starters, a mirage indicates wind direction and speed at your target’s end. Further, the mirage can also cause image distortion making your target appear as if it’s moving in a wavy motion. In such a situation, you’ll want to reduce your scope’s magnification for a clearer image.
Exactly How Much Effect Does Temperature Have on Your Bullet?
Even though the external temperature on a bullet trajectory will remain consistent in the same location, your rifle’s propellant and its level of temperature sensitivity will determine the internal effects.
The table below shows how much of an effect temperature change may have on a bullet during a flight when fired in different environments.
Side Note: The data in this table is derived from a 168gr Hornady bullet and a rifle whose propellant generates a 2.5 muzzle velocity change in every 1-degree change in ambient temperature.
From the table above, it is evident that your bullet’s trajectory flattens as the temperature increases and the air density reduces.
It is also clear that a rifle’s muzzle velocity increases with temperature. The above bullet, fired from the same rifle, will experience a displacement of more than 1 meter when shot from a distance of 900 meters with an air temperature change of 20 degrees Celsius.
You can also see that the difference in bullet drop at 300 meters is almost non-existent — a few centimeters at most. This explains why experienced hunters may not notice the effects of temperatures on a bullet path since they don’t need to account for it to hit targets.
Still, for long distances, gauging the temperature is crucial for accuracy.
What Can You Do to Compensate for Temperature Effects?
Now that you know how temperature affects bullet trajectory, let’s look at how you can correct the situation to ensure spot-on accuracy in hot or cold weather.
Indeed, many sharpshooters and military snipers are taught how to use 1 Minute of Angle (MOA) shift for every 20 degrees change in ambient temperature. However, it would be best to determine how temperature change affects your rifle and cartridge on your own.
The process is pretty straightforward:
Internal Temperature Compensation
To nail how temperature variation affects your muzzle’s velocity, you must determine your propellant’s sensitivity to temperature changes.
You can do this by shooting your rifle through a chronograph and recording the muzzle’s velocity over different temperatures. That way, you can figure out how a 10- or 25-degree Celsius change will affect the velocity.
If you’re using ballistics software to generate trajectory or clicks needed to hit various targets, work with your rifle’s velocity as it would be under the current temperature.
External Temperature Compensation
All you need for this one is a ballistic calculator. Enter the current atmospherics — relative humidity, barometric pressure, and temperature — your calculator will take it from there.
How Does Humidity Affect Your Shooting?
Humidity's effect on your bullet’s flight is relatively slight (as compared to)temperature.
Even though you must account for humidity when shooting over long distances, the trajectory shift — even through drastic humidity changes — is almost always minor and may not require adjusting your rifle’s scope.
Even though temperature and air pressure are far more significant when firing over long distances, humidity is still influential, and you must consider it for accurate shooting.
If you neglect humidity, you may lose accuracy, but you’re still likely to hit your target if you’ve accounted for other factors correctly.
Let’s break down how humidity affects your shooting a little bit further.
Unlike what most people believe, higher humidity causes thinner air. Yes, the air is less dense in high humidity despite how it feels walking into an extremely humid area.
Thus, humidity results in the opposite ballistic effect, contrary to what most would think. So, a bullet will travel easier through humid air.
- Higher humidity causes less dense air and a faster bullet trajectory, delivering a high impact on the target when you don’t account for it.
- Lower humidity results in thicker air, slowing down your bullet’s trajectory and delivering low impact on target when you don’t account for it.
That said, the humidity’s impact on a bullet path is often little unless you’re firing in different environments with varying humidity levels.
How Much Impact Does Humidity Have on Your Shooting?
You now know how humidity affects your shooting, but how much of an impact are we talking about? Does even humidity matter since the impact is negligible?
At close ranges, humidity doesn’t matter. However, the impact is significantly noticeable when shooting targets over longer distances.
The table below shows the variations in bullet drop due to 30% humidity change. This data was derived from a 168grain .308 Winchester bullet.
From the table above you can see that:
- A humidity increase/reduction by 30% at 300 meters, which is inconsequential
- A humidity reduction by 30% at 600 meters causes a 0.6 cm shift in point of impact, which is again inconsequential
- The point of impact shifts by 2.7 cm at 900 meters, which is a significant shift. Still, you can adjust your rifle scope to correct this.
The bottom line is that humidity plays an insignificant role when shooting over distances up to 1,000 meters. What happens for long-range shooting? Here are a couple of examples:
- A 6.5 Creedmoor centerfire rifle cartridge firing a Hornady 140gr bullet over 1,500 meters (about 1,640 yards) will result in a point of impact shift of about 10cm following a 30% humidity drop.
- A drastic humidity change of about 80% would result in a 30cm shift in point of impact for a 0.50cal bullet in vertical displacement.
What does this tell you?
You should only monitor and account for humidity when shooting in a new environment or during seasonal changes. Otherwise, you should focus on monitoring atmospheric pressure and temperature.
How to Account for Humidity When Shooting
You can calculate how humidity affects your shooting using a ballistic calculator. Top ballistic apps for Android and iOS include:
- Strelok Pro
- Ballistic Advanced Edition
Alternatively, you can use a Garmin smartwatch to calculate humidity variations.
Pro Tip: You can use a relative humidity of 50% if you cannot determine the humidity using the solutions above.
The Bottom Line
While temperature and humidity affect your shooting, you can determine the two elements using ballistic solutions. Better yet, platforms like Revic App allow you to measure the effects of temperature and humidity if you’re connected to a local weather station.
And the beauty of it is that it takes a few minutes to get the results.