Cold air is denser than warm air. Duh. This means bullets lose energy as they progress their way through it. What a drag. Literally.
So, a rifle that parks its bullets 4 inches low at 300 yards in 80 degrees (let’s use Fahrenheit for these examples) might put them 5 inches low at -10 degrees. Big deal. An inch of additional drop at 300 yards isn’t going to cost you a buck.
But there’s more. Cold also decreases the “fire” of some smokeless gun powders. Certain powders don’t burn as hot when they’re cold as when they’re already warmed up. Resulting pressure behind the bullet decreases—and so does muzzle velocity. You suffer a double-whammy.
A load that drives a specific bullet 3,000 fps at 80 degrees might push it only 2,900 fps or 2,800 fps at 0 degrees. But how do you know? I’ve yet to see a box of ammunition that included a description of the brand and type of powder used, let alone how it reacts to cold.
So, if you’re concerned about your bullets shooting more slowly and dropping more than usual in frigid winter air, here follows an easy method for determining how much.
Mastering The Math
If you have a chronograph for measuring bullet speed, use it. If not, just pay a bit more attention to your targets. In fact, paper targets alone will tell you what you need to know if you measure precisely and keep careful notes.
First, shoot your loads under controlled conditions with the ammo at a known temperature, say 80 degrees. The easiest way to do this is to store the cartridges in the shade at the rifle range when the ambient temperature is 80 degrees, 70 degrees or whatever you choose.
Fire three carefully aimed rounds from the bench at 300 yards and mark the center of the group. This is your control. Write down the average velocity, too, if you have that chronograph.
Your next chore is to fire another three-shot group at 300 yards, but in much colder temperatures, say 0 degrees or even -20 degrees, if those numbers represent a reasonable temperature for where and when you hunt. Mark the center of this group and record its average velocity. (You can get ammo down to 0 degrees by storing it overnight in a chest freezer and keeping it near that temperature by packing it to the range on ice in a cooler.)
The chronograph velocity readings will tell you how much velocity your powder does or does not lose, and the targets will reveal the added drop effects of cooler-burning powder. To see the additive drag effects of colder, denser air, you’ll have to wait to shoot during a genuine Polar Express cold snap when the outdoors temperature is below zero (or whatever temperature at which you fear you’ll have to hunt.) This is what you really need to know.
Is there enough added bullet drop in extreme cold for you to worry about? You might need to recalculate trajectory for the same load based on shooting temperatures, but chances are you won’t unless you’re targeting small critters at long range. Deer-sized game inside of 300 yards shouldn’t be a problem. Red fox at 300 yards could be.
After you’ve determined hot and cold trajectory differences, make sure you’re actually shooting in or near the temperatures that influence those differences before compensating. If you drive around with your rifle and ammo in a heated truck, spot a buck, jump out into the frigid air for a short stalk and fire before the powder fully chills, it won’t suffer the full effects of the cold. The denser air will still make its impact, but the artificially warmed powder might shoot as hot as it would on an 80-degree day.
If you’re a handloader, you can eliminate the effects of cooler burning powder by loading only those powders that aren’t affected by temperature changes. Several Hodgdon powders fit this profile.
If you shoot factory ammunition only, you’ll need to contact each manufacturer for information on the powders they use in your pet loads or test-fire until you find one that remains consistent.