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Environmental Effects on Bullet Trajectory

James Paine,
December 2, 2022

              We've all been there; those early morning trips to the range in the weeks leading up to the next big hunt. Learning the grouping of your ammunition of choice, figuring out your trajectory and D.O.P.E. (Data Obtained on Previous Engagement), and working out any bad shooting habits you might have. But at the end of the day there is no shot more important than the very first one: the cold bore shot. When you dial in on your chosen prey and line up that shot, you want to have absolute confidence that your bullet will land where you place it. But what factors affect the cold bore shot? And what happens when you end up in completely different conditions than those you had at the range?

              Altitude, temperature, and humidity are three major condition changes to take into consideration when preparing your firearm for a hunt. These factors control the density of the air, or rather, the air pressure. These conditions have varying effects on the bullet, the powder, or both. For example, my home range here in Fairbanks is at 446’ above sea level, but there are places I may get to hunt caribou, sheep, or goat at 2500-5000’ elevation. What if I zeroed my rifle before sheep season opens in early August, when the temperatures in town may be well into the 70s, but I take that same rifle out on a winter caribou hunt and the temperature has dropped to -20F? Some of these changes can be planned for, and others will happen in the moment, but the key is to be aware of the changes and how they affect your weapon.

Author James Paine and friend Jeremy Bingham dialing in shots at higher elevation

              The first of these conditions to consider, and arguably the easiest to plan for in advance, is altitude of the area you plan hunt. Altitude primarily affects the bullet itself as it passes through the air and has little to no direct effect of the powder or rifle. If you’ve ever hiked through country at a higher elevation and had the feeling like you couldn’t catch your breath even during a pause in activity, then you have experienced the exact phenomenon which affects the path of your bullet. The further from sea level you get, the thinner the air becomes; this results in not only less usable oxygen for your body as mentioned, but also a drastic decrease in air density and a much lower drag environment for your projectile to travel through. This means less friction putting resistance on the bullet as it flies, leading to a longer period before it begins to slow down, creating a straighter and smoother trajectory for extended ranges and typically a slightly larger ballistic kill zone.

              The next major condition to consider is temperature. While the bulk of temperature conditions can be roughly guessed by looking at the forecast in your area or understanding seasonal shifts, weather in the mountains can change swiftly and a shift of 40-60 degrees in a single day is far from unheard of. These drastic changes can affect your trajectory in two ways, the first of which is its direct effect on air pressure. Like elevation, the higher the temperature rises, the lower the air density becomes, and vice versa in the colder, lower temperatures where the air pressure will increase exponentially. This happens due to the speed at which the molecules in the air are able to move, just like everything else in the winter, molecules slow down significantly in the cold creating a much denser environment for your bullet to travel through. This adds significant drag to the bullet and the friction will result in your projectile slowing down sooner and having added drop out at extended ranges. Likewise, molecules in warm air move quickly and create less drag on your bullet's path. But, there is still another way temperature affects your bullet's point of impact, or POI for short.

              Most all modern-day factory ammunition loads and powders available to reloaders are variations of white powder, more commonly referred to as smokeless powder, of which there are two varieties: single base and double base. Single base smokeless powder is made from nitrocellulose as its single base accelerant which results in a slower burning powder and is the least sensitive to changes in temperature. Double base powder is made by combining nitrocellulose with nitroglycerine to achieve a faster burn rate and higher pressures in the chamber resulting in higher muzzle velocities, however it is much more sensitive to weather shifts. While all powders show some change in acceleration due to cold weather, double based powders are especially sensitive to it. This is because the added nitroglycerine burn pattern slows down in the colder weather much faster than its nitrocellulose counterpart. The time between ignition of the primer and the individual ignition of each kernel slows down significantly compared to its speed under warm weather conditions. This may only be a change of milliseconds, but when the only time that matters is from chamber to the end of the barrel, milliseconds is all you have and any change in that will create variations in your muzzle velocity.

              The last environmental condition we’re going to talk about is humidity of the hunting area. Humidity’s affect on your point of impact lies 100% on its factor in controlling air pressure, as all modern cased cartridges are sealed and do not allow moisture to contact the powder. Although humidity’s effects may not be near as drastic as temperature and altitude, it still plays a significant role on air density through displacement of the molecules in the air.  When humidity levels rise, evaporated water molecules merge and displace the existing oxygen and nitrogen molecules spreading them out. This results in lower air density as the water molecules weigh less than the oxygen or nitrogen, therefore reducing the amount of friction and resistance on the flight of your projectile.

Madeline Paine zeroing her 30-06 for the fall season

              But how much of a difference will this really make to your point of impact? The answer to that is far too complex to answer in an article alone, as it will depend on your rifle, powder load, environment of the range you zeroed at and that of which you plan to hunt; but I can give you some general guidance on what you can expect and why you should pay attention to these factors.

  • Altitude: The effects of altitude are probably one of the easiest to foresee and adjust for. Barring other factors the absolute air pressure at sea level is 14.70PSI (pound/squared inch). If you were to zero your rifle at sea level in Anchorage, then head out and hunt caribou at 2500’ elevation and then sheep at 5000’ elevation your pressures would be 13.42PSI and then 12.23PSI. At 2500’ your POI is roughly 2” high at 500 yards and 15” high at 1000 yards. Likewise hunting at 5000’ your POI shifts 3” high at 500 yards and 35.25” at 1000 yards.
  • Temperature: Studies have shown that with various popular powders, out of a standard 24” barrel, you can reasonably expect an average of 1 – 2.5 FPS (feet per second) loss per 1°F of temperature drop depending on its sensitivity. From the example given at the beginning of this article, if I zeroed my rifle at 70°F and am now hunting caribou at 0°F, I can reasonably expect a 70 to 175 FPS deviation from my original muzzle velocity depending on my load. This equates to the POI of my 300 Winchester magnum moving anywhere between 4” to 9” below my original zero at 500 yards, and 18.7” to 49.8” low at 1000 yards.
  • Humidity: The factor humidity alone plays on your point of impact is nigh imperceivable to even the best of shooters. For example, if no other factor other than humidity was changed, the difference between 0% and 100% humidity would only change your POI ¼ inch high at 500 yards and 1 ½ inches high at 1000 yards. The point where humidity becomes a factor is when compounded with the other environmental features to influence the atmospheric pressure your bullet must fly through.

As you can see, each of these independently may throw your shot off a decent amount depending on your target size and range. They could also compound each other to have a major impact on your shot or counteract each other to have almost no effect at all. The best thing you can do is simply be aware of these factors and how to account for their impact on your plans of bagging prize game.

              The best and easiest way to account for environmental factors is to have the data you will need ahead of time, and to find and use a trusted ballistics calculator app to interpret that data into usable projections for your specific shooting platform and hunting conditions. These kinds of apps are very easy to use once familiar with the layout and are invaluable in the world of long-range hunting. If you are unsure of your projected conditions and are unable to account for environmental effects on bullet trajectory, keep your shots within close range. Remember, long range shots when hunting are only ethical if you are competent enough to account for all varying environmental factors and can make the shot 10 out of 10 times. Otherwise, it’s a hail Mary and likely not worth the probability of injuring the animal without swiftly dispatching it.

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