Issue 22 A Funny Thing Happened on the Way to the Target Ryan Cleckner Join the Conversation Just Because You Missed Doesn't Mean You Suck You zeroed your rifle and gathered data out to 1,000 yards on a range in Austin, Texas. You just tried using that data in Flagstaff, Arizona, and it didn't work. You missed the target, and you can't figure out why. Why did you miss? Simply put, there are external variables that can affect the bullet's path on the way to the target. If you don't understand the effect these variables have and compensate for them, you might just keep missing. The theory and technique of precision shooting is often explained in an overcomplicated way. It's true that the subject is complex and can take a lifetime to master. However, this article is an attempt to explain the external effects on a bullet in flight in a simple way that shows how they're related. This isn't an advanced course; it's merely an introduction. For all of the examples in this article, we'll use a 175-grain Sierra HPBT bullet fired from a .308 Winchester rifle. You probably have one lurking in the safe, and most people cut their teeth in the long-range game on the good ol' 7.62 NATO. Bottom Line Up Front There are really only two external forces that change a bullet's path on the way to the target, namely gravity and wind. There — you've just gotten all you need to know on the subject. Well done, carry on. Seriously though, if you keep this in mind throughout any discussion of long-range ballistics, you'll have a better grasp of why any factor might influence your bullet. It all comes back to gravity and wind. Both gravity and wind have a greater effect on a bullet the longer the bullet is in flight. Therefore, anything that makes it take longer for the bullet to reach a target will allow gravity to cause the bullet to drop more. It will also allow wind to blow the bullet off of its original path more. It isn't just the target's distance and the bullet's initial speed that determine the flight time of a bullet. Environmental variables, such as air pressure, temperature, and humidity, as well as the bullet's efficiency, can also change how long it takes for a bullet to reach a target. Bullets start to slow down the moment they leave the barrel because of aerodynamic drag. Increased drag, or wind-resistance, means that it'll take longer for a bullet to reach a target, and therefore it can be affected more by gravity and wind. It's important to note that any particular value for these variables doesn't change the path. Instead, a change from one value to another is what has an effect. Also, don't mistake a bullet's speed with a bullet's accuracy. Just because a bullet gets to the target faster does not mean it's necessarily more accurate — it only means that it's exposed to gravity and wind for less time. For example, let's say we have a .308 Winchester rifle with an 18-inch barrel that's very accurate. Because of the short barrel, however, bullets leave the rifle at a slower speed than they would leave a 24-inch barrel. This means that gravity has a longer time span to affect its bullets, and we have to adjust up more for a particular distance target. Even though our bullets drop more, they're still very accurate. Author shooting at Strategic Edge Gun Range in Chapel Hill, Tennessee. Environmental Effects Environmental variables affect the air's density. Denser air causes more drag on a bullet. Decreased temperature or increased air pressure results in denser air. Air pressure (not necessarily altitude) can change the air's density. It is true that higher altitudes generally have less dense air. Weather conditions, however, can change the air pressure at the same altitude and can cause the same air pressure at different altitudes. Therefore, a measure of the actual air pressure is more important than how high you are up a hill. “Station pressure” is the true air pressure at a particular location and altitude. “Barometric pressure,” on the other hand, is a measurement of the air pressure that has been corrected as if the measurement location were at sea level. Barometric pressure is useful when comparing weather conditions in weather reports — station pressure is useful when you are trying to determine the air's density because it's a value that doesn't need to be corrected for altitude. To convert from barometric to station pressure, subtract about 1 inch of pressure for every 1,000 feet of elevation. Of the three environmental factors discussed in this article, air pressure has the greatest effect on air density. For example, the change from a higher station pressure of 30 inches of mercury in Austin to a lower station pressure of 24 inches of mercury in Flagstaff will cause a bullet to impact over 50 inches higher at 1,000 yards because it is getting to the target faster in the thinner (less-dense) air. Air temperature can also change the air's density. Warmer air is less dense. Therefore, higher temperatures result in shorter bullet flight times because of reduced drag. Higher temperatures may also result in faster bullets because gunpowder in a hotter cartridge generally burns faster and produces higher initial bullet velocities. Despite how some powders are marketed, all powders are affected by temperature — some just more so than others. All of this means that both the ammunition's temperature and the ambient air temperature matter. For example, leaving ammunition out in the sun or in a hot chamber can increase the temperature of the cartridge, even if the ambient air temperature hasn't changed. A change from 110 degrees Fahrenheit in Austin to 30 degrees in Flagstaff can cause the bullet to drop 35 more inches at 1,000 yards because it travels slower through the denser air. The effects from temperature and air pressure can partially cancel each other out. At higher altitudes, both the air pressure and the temperature are often lower than at lower altitudes. In the examples above, the lower air pressure causes the bullet to strike about 50 inches high and the lower temperature causes the bullet to strike about 35 inches low. If both the lower air pressure and the lower temperature happened at the same time, then the net effect to the bullet would only be about 15 inches high (the difference between 50 inches up and 35 inches down). Despite most people's intuition, air density actually decreases as humidity increases. As crazy as it sounds, the more moisture there is in the air, the less dense the air is. Therefore, an increase in humidity will result in a faster bullet and a decrease in humidity will result in a slower bullet. Of the environmental factors, a change in humidity has the least effect. For example, a change in humidity from 100 percent to zero would cause a bullet to impact only about 3 inches lower at 1,000 yards, so there's really no need to obsess over it. High Speed, Low Drag The efficiency of a bullet has a direct impact on its aerodynamic drag. A more efficient bullet passes through the air easier and can get to the target faster. A bullet's ability to efficiently move through the air is measured by its Ballistic Coefficient (BC). At its simplest level, the BC of a bullet is calculated by a mathematical model based off of a bullet's density, which is a ratio of a bullet's mass and its cross-sectional area, and its particular shape. The higher the BC, the less drag on the bullet. For example, a Sierra 175-grain HPBT bullet has a listed B.C. of 0.505. If we changed the bullet to a lighter Sierra 168-grain HPBT with a listed B.C. of 0.462, then it will drop 46 inches more at 1,000 yards because it is less efficient and therefore takes longer to reach the target. Even if both bullets leave the rifle at 2,600 feet per second, the more efficient bullet will be traveling over 100 feet per second faster at 1,000 yards. Spin Drift and the Coriolis Effect In addition to the forces and variables discussed above, there are two other things worth mentioning that can change the impact location of a bullet: spin drift and the Coriolis effect. Spin drift is the changing of a bullet's path due to the rotation of the bullet. On a rifle with a right-hand twist barrel (the vast majority of rifles), the bullet will drift to the right. In our example, our Sierra .308 Win. bullet will drift about 5 inches right at 800 yards and 10 inches right at 1,000 yards. This is not a variable that changes — it happens every time, but it does vary between different bullet designs. Horizontal Changes Due to Coriolis Effect Contrary to popular belief, the Coriolis effect is not about the target moving with the surface of the Earth as it rotates below a bullet in flight. Although the rotation of the Earth causes the effect, the deflection of a bullet off of its original course is a consequence of inertia and centrifugal forces. Horizontal changes due to Coriolis depend on your latitude and not on the direction you are shooting. Bullets will always drift to the right in the Northern hemisphere and to the left in the Southern hemisphere. The horizontal effect is greater the further you are from the equator and closer you are to the poles. For example, if you were shooting in Mexico at 20 degrees North latitude, your bullet will drift about 1.5 inches to the right at 1,000 yards. If you were shooting in Alaska at 65 degrees North latitude, your bullet will drift about 4 inches to the right at 1,000 yards. Remember, these shifts will happen regardless of the direction you're shooting. The vertical component to Coriolis, unlike the horizontal changes described above, is greatest at the equator and it changes depending on which direction you are shooting. Shooting directly north or south results in no vertical change. When shooting east with the rotation of the Earth, however, your bullet will impact high and when shooting west your bullet will impact low. For example, shooting east in Mexico at 20 degrees north latitude will cause the bullet to impact the target about 4 inches high at 1,000 yards. If you were shooting east in Alaska at 65 degrees north latitude, the bullet will only impact about 2 inches high. In the Northern hemisphere, spin drift and the horizontal component of the Coriolis effect add to each other. In the examples above, the bullet will drift 10 inches right because of spin drift and about 4 inches right in Alaska because of Coriolis effect for a total of 14 inches to the right. This is, of course, without even considering the wind, which is by far the biggest horizontal influence on a bullet and one that deserves its own article. Vertical Changes Due to Coriolis Effect How Much Does this Really Matter? The short answer is, it depends. If you can't hit a bull's ass with a banjo, then any discussion about external ballistics is pretty academic. First, master proper shooting fundamentals like position and trigger control. Next, worry about gravity and wind's effect on your bullet. Only after you have a good handle on the basics should you start to worry about these other external effects. Clearly, they matter and can cause you to miss when you've done everything else right. However, if you get too wrapped around the details, you might just skip having a proper foundation first. About the Author Ryan Cleckner is a former sniper team leader in the U.S. Army's 1st Ranger Battalion and sniper instructor. Cleckner, who is currently a firearms industry executive, attorney, and university lecturer, has a passion for teaching and explaining seemingly complex topics in a way that anyone can understand. He has appeared as a sniper expert on History Channel's Top Shot program, has a series of popular online firearm instructional videos, and is the author of the Long Range Shooting Handbook. Learn more about him at RyanCleckner.com and NorthShadow.com. 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