February 28, 2023
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.
Now, if you're hunting and your ammo is at 25 degrees, most likely the air temperature will be around 25 degrees, too. This means, in the real world, you must consider both the colder air and the colder ammo. Chart 3 details how these two combined will affect trajectory. As you can see, when the cold air and cold ammo are considered together, you might miss the kill zone of a deer at 400 yards.
As problematic as this might seem at first, some ammunition is hardly affected at all by the cold. This is usually because it's loaded with special powders. In addition to the Winchester load, I also tested a load from Hornady and another from Remington. The 50-degree colder Hornady load was only 7 fps slower than when it was at 75 degrees, and the Remington load was only 29 fps slower. This is not enough velocity loss to matter even if you're shooting at extreme distance.
All these loads were tested in a Nosler Model 21 rifle, and Chart 4 details their performance. It's worth noting that the Hornady and Winchester loads shot measurably better groups when cold than when warm. Both also had less velocity variation when they were cold than when they were warm. The Remington load was slightly less precise but statistically within the margin of error regarding maximum velocity variation and precision on target.
DECIPHERING THE DATA
So, what can we learn from all this data? Well, it's obvious that air and ammo temperatures matter. But it's also obvious that at sensible shooting distances—inside 400 yards—and when using modern cartridges like the 6.5 Creedmoor, it won't matter enough to make you miss a deer.
The challenge comes when you start extending the distance beyond 400—and especially beyond 500—yards. At those distances, large temperature swings can easily cause a miss. You should also keep in mind that these variations go both ways. If you sight in your rifle when it's cold outside and then hunt when it's a lot warmer, your bullet can hit high.
Misses due to temperature differences can also happen when firing at smaller targets where minimal point of impact shifts are more detrimental. A temperature-caused shift of a few inches on a long-range shot at an animal with a smaller vital area like a coyote, for example, could be more consequential.
But, for most hunters in most situations, air and ammo temperatures aren't too concerning. If you're not sending bullets into the next ZIP code, worry more about keeping your feet, hands and body warm so you can hunt longer. A cold, miserable, shivering hunter will have far more impact on accuracy than will cold ammo and cold air. Warm hunters shoot better than cold ones. My advice is to buy warm boots, gloves and clothes for your extreme cold-weather hunts.