How Much Does Cold Weather Affect Bullet Trajectory?
“Cold conditions do have an impact. Let’s see if the difference is enough to matter.
When most hunters work with their rifles in the off-season, they do so when the weather is good, and by good, I mean warm. Even if they test loads when temperatures are more moderate, it’s very possible that when hunting season rolls around it might be much colder. I once deer hunted in Montana when it was 8 degrees below zero, and I’ve had a Saskatchewan deer hunt that was way colder than that! Most shooters have heard that when it’s cold, ammunition performs differently. That’s true, but the big question is: Is the difference enough to matter?
There are two ways that the cold can alter the performance of your ammunition. The first has to do with air density, and the second has to do with the gunpowder. Cold temperatures are the problem with both, but the cold works to increase your bullet’s trajectory in two different ways. So, let’s start with cold air, then we’ll look at cold ammunition, and finally we’ll combine the effects of both to see if it matters enough to make you miss.
When the air is colder, it has more density. This greater density increases drag on your bullet. As an example, let’s look at Winchester’s 125-grain Copper Impact load for the 6.5 Creedmoor. The first line in Chart 1 details the trajectory for this load with a muzzle velocity of 2840 fps at 75 degrees. The second line shows the trajectory for this same load at 25 degrees. The third line in this chart shows the difference between these two trajectories.
It’s clear that the colder air increases bullet trajectory. However, with a difference of only about 2.5 inches at 500 yards, it’s probably not enough to worry about if you’re shooting at a big-game animal. On smaller targets, this may matter more.
When it is cold, gunpowder becomes harder to ignite. This can cause reduced burning temperatures, which translates to reduced pressure. And, when the pressure inside the chamber of your rifle is reduced, velocity is reduced as well. Unlike how we can predict how denser air will impact trajectory, it’s not as easy predicting how the cold will affect gunpowder. This is because some powders are more resistant to temperature variations than others.
To see how drastic this temperature impact might be, I put a box of Winchester’s 125-grain Copper Impact load in the freezer for a week. I then tested some of that load that was at a temperature of 75 degrees alongside the ammo that came out of the freezer at 25 degrees. The results are in Chart 2 and show the difference between the 2840 fps velocity of 75-degree ammo and the 2733 fps velocity of the colder ammo.
The 50-degree difference in temperature caused this load to lose 107 fps. That’s on the extreme of what is common, and at 500 yards, the difference in trajectory could cause your bullet to miss the vital zone of a big-game animal. At closer distances, just as with air density, the difference is not enough to worry about.”
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Photo Credit: Original Author