Most long range shooters are aware that weather conditions affect bullet trajectories, though maybe not by how much or to what degree. In this post, I’m going to take you through all of the effects of temperature, in order of importance. First I’ll talk about the comfort level, and then we’ll discuss the science of how external flight changes. Finally, we’ll look at some sample results from the Sendit Ballistics engine at varying temperatures.
The most important effect temperature has, in my opinion, is shooter comfort. Anyone who has laid in the prone for hours on a cold day knows the difficulty of keeping concentration, and minimizing shivering while making a long shot. On hot days, the shooter’s eyes burn with sweat, and mirage can make the target difficult to see. I have seen square targets at 700+ meters whipping back and forth like a flag through my scope due to the massive mirage. But what do you care about comfort? You’re tough and can deal. You came to read this for the science.
Temperature Effect on Internal Ballistics
Colder bullet powder has less internal energy, resulting in a lower muzzle velocity. The powder combusts more slowly, and simply doesn’t generate as high of pressures as it would on a hot day. The end result is that from freezing cold to burning hot, your bullets may have a variation of 200 feet per second or more on muzzle velocity. This leads to drastically different trajectories. For a more in-depth look at this phenomenon and the history, see a great post on the topic at Firearms History, Technology, and Development. The advanced trajectory package on Sendit Ballistics will account for the internal ballistics variation brought on by temperature changes.
And now, a graph showing the difference in trajectory between two 5.56mm rounds given a muzzle velocity change of 200fps. As you can see, at 1000m, it’s near a 2 meter difference in drop:
Temperature Effects on Bullet Flight (External Ballistics)
By the Ideal Gas Law, we can predict that the air will be less dense as temperature rises. Some quick math:
pressure = density*R*T (T is temp, R is a constant)
density = pressure / (R*T)
Higher temperature leads to lower density. What effect does this have? Let’s check it out from 0 degrees C to 40 degrees C (roughly 32 to 100 F):
It’s a meter or so difference. But not so fast. A change in temperature also leads to a change in the speed of sound. In this case, it’s about a 25m/s difference. That’s important because it means your bullet has a different Mach number at given trajectories. Mach number is important because projectiles experience very different drags at different Mach numbers. Here is the trajectory, only varying speed of sound:
It’s a slight advantage to the cold temperature. Now, for all temperature effects combined: