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The Wind Deflection of Hunting Bullets By Gary Zinn Over the years, I have heard many opinions expressed regarding how wind affects bullet flight. These range from "If the wind is strong enough to ruffle a squirrel's tail, it is too windy to be shooting," to "Bullets fly so fast that there is no time for wind to affect them much." One or both of these old bromides has to be wrong. Computerized ballistic programs can help us cut to the chase, by giving numerical definition to how various wind speeds and directions deflect bullets of given weight, ballistic coefficient and muzzle velocity at various ranges. I ran ballistic analyses on almost two dozen centerfire cartridges and loads and a clear pattern emerged of how wind direction and speed affect bullet flight over practical shooting distances. I can demonstrate the most important things I learned by listing relevant data for two popular cartridges, the .223 Remington and .308 Winchester. The key ways wind affects bullet flight are wind direction and wind speed. Wind direction The wind direction, the angle from which the wind is blowing relative to the direction of bullet flight, is the first key determinant of how much wind of a given speed will deflect the flight of a bullet. This is best explained by reference to the diagram below. Suppose that one is positioned at the center of the diagram, and is shooting at a target at the top (zero degrees). A wind angle of 0 degrees would be a wind that is blowing in the shooter's face (headwind). A wind angle of 180 degrees would be a wind that is blowing at the shooter's back (tailwind). If the wind is blowing directly toward or away from the shooter, it will cause no lateral deflection of a bullet in flight, no matter what the wind speed. A very strong head or tail wind may affect the vertical trajectory of a bullet slightly, but not enough to be of consequence for hunting shots. A wind angle of 90 degrees would be a wind that is blowing on the shooter's right side. A wind angle of 270 degrees would be a wind that is blowing on the shooter's left side. Wind will cause the most lateral deflection of a bullet if it is blowing directly across the bullet path, i.e, from 90 or 270 degrees. The stronger the wind, the greater the lateral displacement; more on this shortly.  Together, the previous two points imply that there is some angle, to either side of a 0 degree headwind or a 180 degree tailwind, within which the wind will not deflect a bullet enough to significantly affect the placement of a hunting shot. Ballistics analysis of a variety of centerfire cartridges and loads leads me to a rule of thumb on this issue: Generally, if the wind is blowing anywhere within 22.5 degrees on either side of zero degrees or 180 degrees, the bullet will not be deflected enough to ruin a normal hunting shot. (22.5 degrees is the angle increment on a 16 point compass rose.) Think of this as defining neutral zones of wind direction, within which one can generally take a normally aimed shot without worrying about wind deflection driving the bullet outside of the vital area of a varmint or game animal. To be clear, this neutral zone is approximately between 337.5 and 22.5 degrees for headwinds, and between 157.5 and 202.5 degrees for tailwinds. This means that the wind directions that may cause significant bullet deflection generally are between 22.5 and 157.5 degrees with a wind blowing right to left, and between 202.5 and 337.5 degrees with a wind blowing left to right. I repeat that this is only a rule of thumb, which does not totally hold for all cartridge loads and shooting distances. I will explore this further in the examples, below. A 360 degree circle may be divided into four sections, or quadrants, for purposes of wind deflection analysis: 22.5 to 90 degrees, 90 to 157.5 degrees, 202.5 to 270 degrees, and 270 to 337.5 degrees. An analysis of wind deflection done for any quadrant (e.g., 22.5 to 90 degrees) can then be applied to the others. For instance, if a 10 mph wind blowing from 22.5 degrees deflects a bullet 3.5 inches at 200 yards, then that result applies for the same speed wind blowing from 157.5, 202.5, or 337.5 degrees. This saves a lot of number crunching when doing the analysis. (There may be insignificant differences in long range bullet deflection at some angles in some quadrants, usually no more than 1/10".) Wind speed Wind deflection of a bullet is a direct, linear function of wind speed. For instance, a wind blowing from a 90 degree angle at 10 mph will deflect a bullet twice as far, at a given distance downrange, as will a 5 mph wind and half as much as will a 20 mph wind. This is convenient, for one may run the ballistics for a particular load at any chosen wind speed and direction, and then determine the amount of bullet deflection at a different wind speed via simple multiplication or division. (This shortcut ignores slight differences due to rounding.) I chose a wind speed of 10 mph for the ballistic analyses below. In my experience, a 10 mph wind is common during hunting seasons, so this is a practical reference for analysis. A 10 mph wind is categorized as Force 3 on the venerable Beaufort Scale, while a 5 mph wind (Force 2) is not of much consequence except for very long hunting shots, while a 20 mph wind (Force 5) is about as strong as is tolerable and manageable while hunting. See the addendum, below, for more information on wind speed and the Beaufort scale. My analysis implicitly assumes that the wind is blowing steadily from a specific direction and at a constant velocity over the distance between the muzzle and the target. Accounting for variations in wind speed and direction over that distance, although common in the field, is way beyond the scope of this article. Two examples My first example of wind deflection analysis is for a .223 Remington load with a 55 grain varmint bullet. Since this is a varmint cartridge and load, I used a +/- 1.5" (3" target diameter) bullet trajectory to determine the maximum point blank range (MPBR) of the load. I used the on-line ShootersCalculator.com programs to calculate MPBRs and downrange wind deflections for all loads listed below. MPBRs and other distances of interest are rounded to the nearest 5 yard increment. Muzzle velocities (MV) are from 24" barrels. I did deflection analyses for a 10 mph wind, blowing at angles of 22.5, 45, 67.5 and 90 degrees to the direction of bullet flight. Of primary interest is how much the wind deflects a bullet at MPBR distance (230 yards) and at what range the wind deflects the bullet enough to drift it out of the 3" diameter target area; i.e., when bullet deflection exceeds 1.5" for this load. I also recorded bullet deflection at 100 and 200 yards. The ShootersCalculator.com ballistic program shows windage results to two decimal places, which I rounded to one decimal place. .223 Remington: Hornady Varmint Express 55 grain V-Max, MV 3240 fps, BC .255, +/- 1.5" MPBR = 230 yards Bullet deflection with 10 m.p.h., 90 degree wind 100 yards: 1.1" Bullet deflection with 10 m.p.h., 67.5 degree wind 100 yards: 1.0" Bullet deflection with 10 m.p.h., 45 degree wind 100 yards: 0.8" Bullet deflection with 10 m.p.h., 22.5 degree wind 100 yards: 0.4" This data clearly shows how wind angle affects bullet deflection. At the extremes, a 10 mph wind blowing at a 90 degree angle will cause deflection of over 1.5" at 120 yards (48 percent less than MPBR). The same wind at a 22.5 degree angle will deflect a bullet more than 1.5" at 190 yards (only 17 percent less than MPBR). Bottom line: acute wind angles and significant wind speeds mean that windage is an important factor to be dealt with when plinking at varmints with a .223 Remington, or similar cartridges. Another important thing shown by the data is how rapidly windage increases at longer ranges. With a 10 mph, 90 degree wind, bullet deflection increases from 4.6" at 200 yards to 6.2" at 230 yards. This is a 35 percent increase in windage over a 15 percent increase in range! This observation leads to another interesting rule of thumb: Generally, half of the distance that a bullet will be deflected by the wind at MPBR range occurs within (approximately) the last 30 percent of the distance between the muzzle and MPBR. This says much about the challenges of making accurate hunting shots at longer ranges under windy conditions. Earlier, I suggested the rule of thumb that 22.5 degrees either side of zero or 180 degrees is a windage neutral zone, in the sense that a wind blowing at those angles generally will not deflect a bullet enough to require lateral sighting adjustments at normal shooting ranges. This is not true in this particular case, because a 10 mph wind blowing at a 22.5 degree angle will drift a bullet 2.4 inches at 230 yards. I did another ballistic run for this load and found that its actual neutral zone angle is about 15 degrees. A 10 mph wind blowing from 15 degrees will deflect a bullet 1.6" at 230 yards. In this case, the 22.5 degree neutral zone rule of thumb would hold for a wind speed of 6.25 mph or less. Obviously, my 22.5 degree rule of thumb is just that, and is not to be taken as gospel, particularly for varmint loads. The .308 Winchester cartridge ranks second on the latest Guns and Shooting Online list of The 10 Best Selling Centerfire Rifle Cartridges in the USA. (The .223 Remington ranks first.) The .308 Winchester is doubtless most used for hunting deer in the US and a 150 grain bullet is the load of choice for most deer hunters. Ballistic analysis of this cartridge is based on the same parameters as were used for the .223 Remington, except that bullet trajectory is for a +/- 3" MPBR (6" target diameter); this yields a 275 yard MPBR for the load. .308 Winchester: Hornady Custom 150 grain SST, MV 2820 fps, BC .415, +/- 3" MPBR = 275 yards Bullet deflection with 10 m.p.h., 90 degree wind 100 yards: 0.8" Bullet deflection with 10 m.p.h., 67.5 degree wind 100 yards: 0.8" Bullet deflection with 10 m.p.h., 45 degree wind 100 yards: 0.6" Bullet deflection with 10 m.p.h., 22.5 degree wind 100 yards: 0.3" This load bucks the wind quite well. A 10 mph, 22.5 degree wind will deflect a 150 grain bullet only 2.4" at 275 yards, so my 22.5 degree neutral zone rule of thumb holds in this case. With a 45 degree wind, bullet deflection exceeds 3" at 230 yards (16% less than MPBR), while the over 3" deflection distances with 67.5 and 90 degree winds are 25% and 29% below MPBR. A .308 Winchester firing 150 grain loads has been my go-to deer rifle for many years. Early on (before on-line ballistic programs), I decided that I would limit my shooting distance to no more than about 150 yards whenever I was contending with a significant cross wind. The data above show that this self-imposed range limit is, in fact, somewhat conservative for cross winds of 10 mph or less. This does not mean that I am going to change my practices, however, because I like my chances with 150 yard or shorter shots much better than with longer shots, regardless if there is a strong wind blowing. Velocity, bullet weight and ballistic coefficients Anyone who hunts a lot in strong wind conditions can make their life simpler by choosing cartridges and loads that resist lateral bullet deflection. This involves choosing a cartridge and load that best balances bullet velocity, weight and ballistic coefficient. Here is an example. A .30-06 Hornady Superformance 165 grain SST load has a MV of 2960 fps; the bullet in question has a BC of .447. The ballistics for this load, when fired through a 10 mph, 90 degree wind are +/- 3" (6" target diameter) MPBR = 290 yards Bullet deflection with 10 m.p.h., 90 degree wind 100 yards: 0.7" The corresponding data set for the .308 Winchester load above (150 grain SST bullet, BC .415, MV 2820 fps) was +/- 3" MPBR = 275 yards. Bullet deflection with 10 m.p.h., 90 degree wind 100 yards: 0.8" The .30-06 load, with a heavier and higher BC bullet, driven at higher velocity, resists wind deflection better than does the .308 load. With a 10 m.p.h., 90 degree wind, the .30-06 load carries 15 yards further than the .308 load before wind deflection exceeds the target area radius and the .30-06 load has nearly an inch less deflection than the .308 load at the MPBR range of the latter. Both loads have wind deflection of less than 3" at MPBR range with a 10 mph, 22.5 degree wind angle (2.4" at 275 yards for the .308 load, 2.3" at 290 yards for the .30-06 load). The ballistic differences between these loads illustrate the point that a cartridge/load combination that has the attributes of high velocity, relatively heavy bullet weight and high bullet BC will buck the wind better that a slower, lighter weight, lower BC cartridge/load. Conclusion I do not much believe in making the taking of hunting shots into a technical exercise. Therefore, I do not plan to carry a wind meter and table of bullet deflection data into the deer woods. What I have learned from this exercise is that hunting shots more or less directly into (or with) the wind can be taken without worrying about wind speed. (I knew this intuitively, but the data confirm it.) However, if a strong wind is blowing at more than about 22.5 degrees off of the line of fire I need to subjectively evaluate the wind speed, angle and the distance to my target, so that I can determine whether the range is short enough to aim right on, or if I need to hold "into the wind" a few inches to assure a vital area hit. Studying the ballistics analysis tells me how far winds of given speeds, blowing from particular angles, will deflect the bullets from my cartridge/load, which helps me make this judgment call. Obviously, I could not include ballistic analyses in this article for the many game and varmint cartridges and loads that are available. Rather, my purpose was to demonstrate how one may go about working out and interpreting the analysis for any particular cartridge/load of interest. Addendum: the Beaufort wind force scale This summary of the Beaufort Wind Scale is based on information from the NOAA and Wikipedia websites. The scale is named for Sir Francis Beaufort (U.K. Royal Navy), who developed it in 1805. Editorial comments I have added are denoted by brackets. [ . . . . ] Force 1 - Light air (1 - 3 mph): Direction shown by smoke drift, but not by wind vanes. [No significant deflection of bullets at normal hunting ranges.] Force 2 - Light breeze (4 - 7 mph): Wind felt on face, leaves rustle, wind vanes begin to move. [Frequently encountered during hunting seasons. Significantly deflect high velocity bullets only at very long range.] Force 3 - Gentle breeze (8 - 12 mph): Leaves and small twigs in constant motion; light flags extended. [Wind conditions at this level are common during hunting seasons.] Force 4 - Moderate breeze (13 - 18 mph): Dust, leaves and loose paper lifted, small tree branches move. [Unpleasant hunting conditions.] Force 5 - Fresh breeze (19 - 24 mph): Small trees in leaf begin to sway. [Very unpleasant hunting conditions, plus game is likely to be holed-up.] Force 6 - Strong breeze (25 - 31 mph): Large branches in motion, whistling in wires, umbrellas used with difficulty. [Not a good time to be hunting.] Force 7 - 12 (32 mph and greater): Includes hurricane force winds, which start at 74 mph for a Category 1 hurricane. [Force 7 and higher winds mean it is way too windy to be hunting.] |