Fig. 1 The Kestrel portable weather station is standard equipment among snipers. The JTF2 team that scored a world record in 2017 would have carried them. They provide temperature, wind speed, humidity, barometric pressure, and other metrics. Atmospherics are critical for ELR shooting. The latest Kestrels have built-in ballistic calculators, though probably not as sophisticated as the team’s proprietary software.
The World Record Sniper Kill
For a detailed analysis of the Mosul world record shot, see The Perfect Long-Range Kill and Science of the Long-Range Kill. The first deals with holdover, the second with stability and spin drift. This article continues with the Coriolis adjustment.
In 2017, a pair of Canadian JTF2 sniper teams was operating from a firing position on the east bank of the Tigris River on the north side of Mosul. ISIS fighters were in action against Iraqi police on the other side of the river, two miles away. The snipers were located on an upper floor of a hotel and had a comprehensive view of the battle space on the West Bank.
The snipers made a world-record kill shot at a range of 3,540 meters (2.2 miles). That range converts to 3,871 yards. I gave a fairly detailed analysis of the shot in the article, The Perfect Long-Range Kill. That article focused mainly on the ammunition used (the Raufoss .50 caliber API) and the holdover. In a later article, Science of the Long-Range Kill, I discussed bullet stability and spin drift.
In this article, I will discuss the Coriolis Effect and its adjustment for a two-mile shot.
The Coriolis Effect
The world turns. You wouldn’t notice it standing there, because the Earth is so huge, but it does revolve around its axis. Let’s say we fire a Minuteman ICBM at Moscow. It’s a thought experiment, an example. The missile lifts off and flies toward its target on a ballistic trajectory – just like an artillery shell or a bullet. But the Earth is turning. If we aimed our missile directly at Moscow, and the Earth turned beneath the missile, the shot would miss. Our missile forces, whether they are located in underground silos or ballistic missile submarines, have to take the movement of the Earth – the Coriolis Effect – into account when calculating their firing solutions. We have to do the same.
Figure 2 shows the deflection experienced by objects detached from the surface of the Earth as the world turns beneath them. ICBMs, bullets, footballs, and even airplanes are affected. Their courses must be corrected.
The World Record Sniper Kill
For a detailed analysis of the Mosul world record shot, see The Perfect Long-Range Kill and Science of the Long-Range Kill. The first deals with holdover, the second with stability and spin drift. This article continues with the Coriolis adjustment.
In 2017, a pair of Canadian JTF2 sniper teams was operating from a firing position on the east bank of the Tigris River on the north side of Mosul. ISIS fighters were in action against Iraqi police on the other side of the river, two miles away. The snipers were located on an upper floor of a hotel and had a comprehensive view of the battle space on the West Bank.
The snipers made a world-record kill shot at a range of 3,540 meters (2.2 miles). That range converts to 3,871 yards. I gave a fairly detailed analysis of the shot in the article, The Perfect Long-Range Kill. That article focused mainly on the ammunition used (the Raufoss .50 caliber API) and the holdover. In a later article, Science of the Long-Range Kill, I discussed bullet stability and spin drift.
In this article, I will discuss the Coriolis Effect and its adjustment for a two-mile shot.
The Coriolis Effect
The world turns. You wouldn’t notice it standing there, because the Earth is so huge, but it does revolve around its axis. Let’s say we fire a Minuteman ICBM at Moscow. It’s a thought experiment, an example. The missile lifts off and flies toward its target on a ballistic trajectory – just like an artillery shell or a bullet. But the Earth is turning. If we aimed our missile directly at Moscow, and the Earth turned beneath the missile, the shot would miss. Our missile forces, whether they are located in underground silos or ballistic missile submarines, have to take the movement of the Earth – the Coriolis Effect – into account when calculating their firing solutions. We have to do the same.
Figure 2 shows the deflection experienced by objects detached from the surface of the Earth as the world turns beneath them. ICBMs, bullets, footballs, and even airplanes are affected. Their courses must be corrected.
Fig. 2 Illustration of how objects detached from the Earth’s surface are deflected in the northern and southern hemispheres by the west-to-east rotation of the Earth.
The Coriolis Effect doesn’t matter under 600 yards. In fact, the calculated adjustment at that range is probably so small it wouldn’t amount to a click-stop on your scope. Over longer distances, it does matter. And it would have, that day in Mosul. So let’s talk about Coriolis. Look at Figure 3.
Fig. 3. Northern hemisphere Coriolis Effect on a bullet’s Point of Impact (POI) when firing directly north from Point A to Point B. The shooter will miss right whether he is firing from Point A to Point B, or from Point B to Point A. The effect is opposite in the southern hemisphere.
Horizontal Coriolis
In Figure 3, north is given by the arrow extending from Point A to Point B. Imagine yourself driving at 55 mph directly east from Point A. You’re driving in the direction of the Earth’s rotation. Point B is stationary. You point a rifle at Point B and fire. Because you are moving and so is the bullet, you will miss to the right. This is also true if Point B is another car driving east, slower than you are. You’ll have to adjust your aim to the left if you want to correct for the deflection.
Now imagine yourself standing stationary on Point B while a car drives past from Point A, headed east at 55 mph. You aim at the car and fire. Once again, you’ll miss to the right and will have to correct left. This is true even if you are in a car driving east at 20 mph.
The above is true in America. If you perform the same exercise in Australia, you’ll find the opposite is true. You will always miss to the left and have to correct to the right. Look at the right-hand globe in Figure 2. The cannon fires a shell in the southern hemisphere that deflects left. Correction requires a sight adjustment to the right.
The Encyclopedia Britannica explains this phenomenon somewhat more technically: “There are two reasons for this phenomenon: first, Earth rotates eastward; and second, the tangential velocity of a point on Earth is a function of latitude (the velocity is essentially zero at the poles and it attains a maximum value at the Equator).” For this reason, the calculation of a Coriolis adjustment requires knowledge of your latitude. If you are standing closer to a pole, you are moving slower than if you were standing closer to the Equator.
Corrections for horizontal Coriolis Effect are either added to or subtracted from your adjustments for windage and spin drift.
Vertical Coriolis
You’re getting the idea. If we’re standing on Point A and facing due east, we’ll see a similar effect. This time, no horizontal adjustment will be necessary. However, if we aim at an IPSC head-and-torso target’s face, we’ll miss over the top. We’ll have to aim chest level to hit the target’s face.
If we turn around and set up a similar target to the west, we’ll find the opposite. If we aim at the target’s face, we’ll hit it in the chest. We’ll have to hold over a similar amount to correct for the drop.
At the Equator, no horizontal Coriolis adjustment is necessary.
The video below demonstrates that the effect of vertical Coriolis is material at 1,000 yards and provides a graphic example of the phenomenon described:
There are trigonometric formulas available to calculate both horizontal and vertical Coriolis adjustments. The shooter needs his latitude and a calculator with trig functions. If one has a programmable calculator, one could easily stick in the formulas. The Kestrel 5700 Elite is a portable weather station with a ballistic calculator that incorporates the Coriolis Effect.
I’m keeping this series math-lite, so I will not include the formulas. If anyone is interested, let me know.
Fig. 4 Kestrel 5700 Elite portable weather station with ballistic solver. For Extreme Long Range shooting, evaluating inputs is critical. There is no one solution – for example, wind can be different at the target location as compared to the shooter’s location.
The Coriolis Effect Mattered
What about the 2-mile sniper kill – did the Coriolis Effect matter?
Short answer is yes. The vertical Coriolis was about 1.0 MRAD (3.4 MOA), and the horizontal Coriolis was about .4 MRAD (1.4 MOA). Given the range of 3,871 yards, this would translate to a Coriolis holdover of about 11 feet and a Coriolis holdoff of about 4.5 feet to the left.
The horizontal Coriolis adjustment was in the same direction as the spin drift and wind corrections, so the total adjustment required was quite large. The vertical Coriolis adjustment was also in the same direction as the drop due to range, but constituted a much smaller percentage of the total adjustment.
Conclusion
Inside six hundred yards, the Coriolis Effect isn’t worth worrying about. As we’ve pointed out, it probably doesn’t even rate a click on your turret. Beyond that, it’s worth being aware. At two miles, it’s the least of your worries, but that’s not to say it can be ignored.
The world record shot had a total holdover of 1,122 feet, of which the Coriolis component is 11 feet. Ignore it, and you will hold over 1,111 feet. Your bullet will hit eleven feet low and kick up dirt at the ISIS fighter’s feet.
About the Author
Cameron Curtis
You may reach Cameron at: cameron.curtis545@gmail.com
Cameron Curtis has spent thirty years in the financial markets as a trader and risk manager. He was on the trade floor when Saddam’s tanks rolled into Kuwait, when the air wars opened over Baghdad and Belgrade, and when the financial crisis swallowed the world. He’s studied military affairs and warfare all his adult life. His popular Breed series of military ad-venture thrillers are admired for combining deep expertise with propulsive action. The premises are realistic, the stories adrenaline-fueled and emotionally engaging.
Book 11, Death Hunt, is now available. For a limited time, all books are priced at $0.99 each:
As someone who’s seen what happens when the truth is distorted, I know how unfair it feels when those who’ve sacrificed the most lose their voice. At SOFREP, our veteran journalists, who once fought for freedom, now fight to bring you unfiltered, real-world intel. But without your support, we risk losing this vital source of truth. By subscribing, you’re not just leveling the playing field—you’re standing with those who’ve already given so much, ensuring they continue to serve by delivering stories that matter. Every subscription means we can hire more veterans and keep their hard-earned knowledge in the fight. Don’t let their voices be silenced. Please consider subscribing now.
One team, one fight,
Brandon Webb former Navy SEAL, Bestselling Author and Editor-in-Chief
Cameron Curtis has spent thirty years in the financial markets as a trader and risk manager. He was on the trade floor when Saddam's tanks rolled into Kuwait, when the air wars opened over Baghdad and Belgrade, and when the financial crisis swallowed the world. He's studied military affairs and warfare all his adult life. His popular Breed series of military adventure thrillers are admired
Barrett is the world leader in long-range, large-caliber, precision rifle design and manufacturing. Barrett products are used by civilians, sport shooters, law enforcement agencies, the United States military, and more than 75 State Department-approved countries around the world.
PO Box 1077 MURFREESBORO, Tennessee 37133 United States
Scrubba Wash Bag
Our ultra-portable washing machine makes your journey easier. This convenient, pocket-sized travel companion allows you to travel lighter while helping you save money, time and water.
Our roots in shooting sports started off back in 1996 with our founder and CEO, Josh Ungier. His love of airguns took hold of our company from day one and we became the first e-commerce retailer dedicated to airguns, optics, ammo, and accessories. Over the next 25 years, customers turned to us for our unmatched product selection, great advice, education, and continued support of the sport and airgun industry.
COMMENTS
There are
You must become a subscriber or login to view or post comments on this article.