The importance of accurate range calculation is never so great as when shooting at extreme distances. The farther your bullet travels, the more its trajectory becomes a plunging arc, where even tiny errors grow into major significance. A range estimation error of only 5 percent means little at 400 yards, but at 1,500 yards that puts your .50-caliber projectile's impact point about 8 minutes long or short. Your bullet will sail a dozen feet over your target's head or thud harmlessly into the ground yards short of him. A long-range laser allows precision measurement, but for extreme distances that device must be aimed as carefully as a rifle, with keen awareness to avoid lazing beyond or before tile target's actual location.
Wind estimation, too, gets tricky at extreme distances. Try to monitor the wind at three or four points between you and the target to ensure you don't overlook contradictory cross-winds. Observing falling rain or snow helps to identify such winds.
Correctly interpreting the effect of distant oblique winds becomes critical as well. For instance, when firing 709-grain M2 ball at 1,500 yards, a relatively mild 5 mph wind mistakenly judged to be coming across at 1:30 o'clock but actually crossing at 2 o'clock will cause a lateral compensation error of 18 inches—quite likely a complete miss.
Extreme-range shooting opens us to all sorts of ballistic concerns and factors that had seemed only esoteric at shorter distances. Due to your bullet's considerable time of flight, you must attune yourself to interpreting human visual cues to ensure that your target remains stationary long enough for the bullet to reach him. At 1,500 yards, a .50's flight time is 2.3 seconds, plenty of time for a sitting man to stand or a stationary man to take a step. In order to shoot where he is—not where he was— you must time your shot.
Time of flight also relates to the rotational speed of the earth. In a single day—one earth rotation—the planet turns approximately 25,000 miles at the equator, which equates to 1,042 mph, or 1,531 fps, with slightly less relative speed as you approach the poles. Your bullet's speed will vary a tiny amount, depending on whether you're shooting with or against the earth's rotation or angled away from the equator. It's a tiny, tiny amount, but its influence, too, grows with distance.
Likewise with the Coriolis effect. In 1835, Gustave de Coriolis, for who the effect is named, demonstrated that the Earth's rotation affects winds and ocean currents and even dictates whether your toilet flushes water clockwise or counterclockwise. Like a curveball leaving a pitcher's hand, your bullet is slightly affected by the Coriolis effect. It's not enough to matter at 1,000 yards or less, but its influence grows with range.
Theoretically, a headwind slightly increases drag on your bullet and thereby reduces its velocity, while a tailwind has the opposite effect. Thus, a headwind requires that you raise your elevation slightly, and a tailwind dictates that you lower it slightly. The formula for calculating the required compensation, according to 19th-century firearms authority W.W. Greener, is:
Wind Velocity (MPH) x Distance
(Hundreds of Yards) yards of
4 (Math Constant) ~ Range Change
Using Greener's formula, let's calculate the compensation for a tailwind of 20 mph when engaging a target at 1,550 yards.
(Hundreds of Yards) 310
4 (Math Constant) 4
Therefore, reset your elevation 77.5 yards closer, as if the target were at 1,422.5 yards, but do so with caution. Consider this: there's no variance in Greener's formula for a constant wind pushing on your bullet's full trajectory and a wind that affects it during only part of its flight. That's pretty substantial.
Air pressure matters, too, for the denser air at low altitude slows your bullet and the thinner air at high altitudes allows your bullet to fly faster. Then there's temperature, especially ammo temperature, because warmer gunpowder burns a bit faster than cooler gunpowder, with a resulting variance on muzzle velocity.
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