How Weather Affects Accuracy

How Weather Affects Accuracy

By Troy Hoepker

Many of us seek out a day to sight in our rifle when winds are calm, temperatures are warm and comfortable, and after being pleased with how the rifle shoots, happily put it away until that first all-important day that we hunt in the fall or winter. I know I’ve done it and venture to guess that many of you have also done the same. We expect to have the same results when we fire at an animal but usually the day we hunt is completely different than the day we sighted in. The rifle and round are subjected to colder temperatures, perhaps wind and maybe even precipitation. What happens to our point of impact and do we know how the elements will change it?

When we think of weather (and for the purpose of this article) we will include: temperature, humidity and precipitation, wind, light, barometric pressure and elevation as factors that influence ballistics. Many of the influences of these factors are minor when discussing typical Iowa hunting conditions where elevation and distance of the shot are minimal but there are some things associated with each weather condition that may influence your rifle’s performance.

Temperature
The degree to which the flight of a bullet is impacted by temperature is heavily dependant on the distance of the shot. For all practical purposes it is not something we have to take much consideration for unless we are possibly involved in extreme long range shooting. Warm air is thinner or less dense than cool air and as a result, a bullet will encounter less resistance through the air on a warm day than it does on a cooler day. Something to keep in mind if you have sighted your rifle in at a range such as 400 yards is that if you did so on a warm day, a shot of the same distance on a cold day would experience more drop than you might be expecting. You’ll likely see no discernable difference at shorter ranges such as 100, 200, or maybe even 300 yards but it is wise to know the effect colder air has on the ballistics especially when we factor in something that may have even more affect on your point of aim and that is the burn rate of your powder and heat of the firearm. For instance, the initial temperature of the propellant or gunpowder has a significant affect on the burn rate. The energy to push against a bullet is created by the sudden change of a solid into a gas.

Propellants are very poor conductors of heat. So when a cartridge is suddenly exposed to cold temperatures, there exists a higher likelihood of a slower burn rate or even the possibility of miniscule particles of the charge that don’t completely burn, thus resulting in lower velocities than you might have experienced while test firing in warmer conditions. Conversely it’s also an extremely good idea to keep ammunition stored in places away from outside sources of heat or direct sunlight. Ammunition stored in extreme heat can produce varied burn rates and become unreliable. Some powders are more susceptible than others to temperature changes.

Our barrel temperature also affects performance. On a cooler day the outside air is able to absorb more heat from the barrel than on a hot day of say 80 or 90 degrees. This helps keep the barrel cooler resulting in less baking or fouling inside the barrel while firing multiple shots resulting in a loss of accuracy. Temperature also plays a huge role in how our rifle functions. Extreme cold can play havoc on moving parts of a firearm. Any lubricant leftover in a firearms action has the ability to freeze rendering the gun inoperable. So in extreme cold it’s a good idea to have a dry action. Air contains moisture and as it is cooled it collects on surfaces and so when you bring a cold rifle into a warm environment, moisture will collect all over the gun. Not a big problem other than rust until you take that rifle back out into the cold once more and the moisture freezes. In an AR-15 style rifle a frozen action could cause enough backpressure in the gas tube to permanently damage the gas block. So it’s a good idea to wipe down any firearm that has been exposed to the elements so you hear it go bang instead of click the next time you pull the trigger.

Humidity and Precipitation
A water molecule weighs less than a dry air molecule. Moist air is actually less dense than dry air. Most shooters would assume the opposite and theorize that a bullet in the air on a very humid day would experience more resistance than one going through dry air. The biggest thing to worry about when it is humid is your firearm and preventing rusting.
The same is true when shooting in precipitation. One would think that a bullet being impacted by water while in the air would have a detrimental effect, but it actually has little to no effect on projectiles. Rain generally occurs during low barometric pressure readings and lower pressure means less air resistance because of less density. The air is thinner. In heavier rains loss of accuracy is usually because of accompanied heavier winds or increased discomfort of the shooter obviously.

Elevation and Barometric Pressure
Elevation and barometric pressure affects the density of the air and therefore the amount of resistance it exerts on the bullet as it travels to the target. Air density is a direct multiplier in the equation for the drag force on a bullet and because air densities decrease rapidly as altitude increases, altitude has by far the largest effect on a bullet’s trajectory compared to actual atmospheric conditions. Effectively, the bullet acts as though it has a higher ballistic coefficient as altitude increases and is able to travel in flight flatter for longer distance. Barometric pressure is the pressure exerted by the weight of air in the atmosphere. At higher elevations there is less overlying atmospheric mass generally speaking, unless a front is moving in. But for all practical purposes here in Iowa we don’t have to worry much about these variables, including atmospheric pressure, unless we are planning a hunting trip to a mountainous region. In that case it would be wise to sight in your rifle once you get to the elevation you will be hunting in. In Iowa the effects of barometric pressure are negligible because most of the time a higher than normal temperature tends to be accompanied by a higher than normal barometric pressure of the atmosphere. So a high temperature (warm day) tends to decrease air density, while high pressure tends to increase air density in effect revealing a relationship between the two that in most cases cancels out the effect of the other. In extreme long range shooting such as 1,000 yards it may be beneficial to calculate the effects of your elevation and the barometric pressure.

Light
How does light matter you may ask? The amount of light illuminating your target can be the difference in seeing the point of aim clearly and not seeing it clearly leading to a misplaced shot. Your point of aim can change when shooting long distances while dealing with a change from sunlight to overcast skies or shadows. If you are sighted in at a target that is shaded and suddenly the target becomes illuminated more brightly by the sun, at 1,000 yards, you would now shoot low at the same target when trying to hit it. The opposite is true as well. If you were sighted in properly on a bright, well-lit target and a cloud came overhead, you will now hit high on the shadowed target. Bright sunlight also tends to increase the amount of heat mirage you might see through your scope as you try and acquire the point of aim making it more difficult to see the exact spot you want to aim at.

Wind
Wind is probably the number one influencing factor on rifle accuracy that most of us deal with. Judging the wind and how much it will push your bullet from one side or the other for the distance involved is not always easy.
A good first step in having a bullet perform as well as possible in wind or no wind for that matter, is selecting one that has a high ballistic coefficient. A bullet’s ability to retain as much muzzle velocity as possible is an important factor in both trajectory and game killing effectiveness. The bullet’s ability to retain its stability in the air because of good balance and proper weight, diameter, and shape are the determining factors that govern ballistic coefficiency. The ballistic coefficient is a measure of how well a bullet can overcome air resistance and maintain flight velocity. A good bullet design has strong aerodynamic aspects that include several important things; the parasitic drag, or the amount of friction of air molecules as they slide along the outer edge of the bullet; the bow wave, which is the physical wave created in air as the nose of the bullet travels at supersonic speed; front air compression or the amount of compressed air in front of the nose of the bullet as it travels; the partial vacuum, or how air collapses around the back of the projectile. The ballistic coefficient and velocity are the main factors in cutting wind deflection.

If a heavier bullet and a lighter bullet are fired at the same velocity the wind will deflect the heavier bullet the least, all other things being equal. If the pressure level of the charge is kept equal between the two, there will be similar deflection of the two bullets. The heavier bullet can buck the wind better but the lighter bullet will have more velocity. If two bullets are identical, including the same ballistic coefficient, the bullet fired at the faster velocity will be deflected by the wind the least. Many hunters and shooters believe that it is best to choose a heavier bullet in windy conditions because it won’t be pushed off course as far as a lighter one. While that can be true, the above example shows that there is more to it than just selecting a heavy bullet believing that it will deflect the least. A higher velocity decreases the amount of time the bullet is in the air and the more time a bullet is in flight between muzzle and impact, the more exposure it has to the wind and the more it will be blown off course. However with long-range shots, a lighter bullet fired at a faster velocity may not hold its velocity as well in flight as a heavier bullet in a crosswind. Once the lighter bullet’s momentum drops in its trajectory it can lose its advantage over a heavier bullet. So it’s important to know the average distance of a shot you normally take when hunting when choosing a windy day bullet.

Estimating wind speed is tough. Between you and the target you may have wind speeds of varying amounts and even varying directions working on the projectile’s flight path. But there are a few things that may help with gauging the correct amount of windage to use before pulling the trigger. One thing some folks don’t consider is that a wind close to the shooter will cause much more deflection than a wind that is only close to the target. This is something you may very well encounter in a hunting situation when you are elevated and in the open shooting at an animal that may be situated naturally out of the wind. As soon as the bullet leaves the muzzle and is hit by the wind it quickly begins a lateral momentum or deflection and once it has started, it will move off more and more even if it encounters calm air thereafter. Conversely, if you are shooting from a calm area such as in the woods at a game animal in the open where the wind only affects the last bit of the flight path, it is not necessary to adjust nearly as much for the wind.

Making the correct minute of angle adjustments in wind is a matter of experience and intuition, adjustment from shot to shot and using the surroundings to gauge wind speed such as tall grass, leaves or small branches. Also estimating the angle of crosswind is beneficial. Imagine a clock and try to determine the angle of wind you are dealing with. A wind coming from the right at 90 degrees would be 3 o’clock and a wind from the left would be 9 o’clock for example. In military terms a wind from 6 and 12 o’clock positions have no value while winds from 2, 3, 4, 8, 9 and 10 o’clock are called full value meaning they are heavily influenced by a crosswind. Winds from the 1, 5, 7 and 11 o’clock are called half-value because of lesser degrees of push on the bullet’s path.

Above all, the best thing you can do to get better at “wind doping” is go out and shoot when it’s windy. Grab some targets and shoot from different distances and at varying angles to the wind while your hat is blowing off. If you reload you can even try some different bullets and powder charges to see what might handle the wind the best out of your gun.

Weather and the elements affect accuracy in so many ways, some more noticeable than others. Take a few of these tips and facts as knowledge to store in your memory bank for those days when it’s not perfect outside. Live in Iowa very long and you’ll find there are more days of inclement weather of some kind than there are days suited ideally for shooting.