caused by a 10-mph crosswind can be achieved by upgrading the ballistic coefficient. For example, the 0.155 ballistic coefficient curve, when intersected by horizontal lines representing the deflections equivalent to the 3X width of ground squirrels (GS), prairie dogs (PD), and woodchucks/rockchucks (WC) indicates that at distances in excess of 190, 220, and 250 yards, respectively, the deflections will exceed the 3X widths of these varmints. If the ballistic coefficient is increased to 0.275, the distances at which wind deflections become greater than the 3X widths ap- proximate 260, 315, and 360 yards, respectively, an increase of roughly 70 to 110 yards in the effective shooting distance. An additional 30 to 40 yards is gained if the ballistic coefficient is approximately 0.420. The advantage of higher ballistic coefficients becomes even
more apparent when the crosswind velocity increases beyond 10 mph. For example, in a 20-mph crosswind, a 22-caliber bullet with a ballistic coefficient of 0.420 and a muzzle velocity of 3,300 fps will deflect approximately 12 inches at 300 yards, whereas a 22-caliber bullet with a 0.155 ballistic coefficient at the same muzzle velocity will be deflected approximately 41 inches. Judg- ing deflection corrections for shots at small critters in crosswinds exceeding 10 mph becomes a guessing game beyond 350 to 400 yards, even with ballistic coefficients greater than 0.400. It is worth noting that while lead-free bullets are receiving
Wind deflection curves for each of the five groups of average varmint cartridge/bullet ballistic coefficients. The horizontal lines representing the 3X widths of woodchucks/rock chucks (WC), prairie dogs (PD), and ground squirrels (GS) intersect the curves to define the maximum distances within which 10-mph crosswind deflection is less than three times the width of the designated varmints. By using bullets with higher ballistic coefficients, it is possible to minimize wind deflection and significantly increase the yardage for high-percentage shots, as represented by the wide shaded bands extending between the curves.
ues of more than 9 inches at 300 yards, making these cartridges/ bullets questionable choices for the varmint hunter planning to undertake long-range shots at prairie dogs on the western plains where wind speeds routinely average more than 10 mph. Also, as shown, the cartridge/bullet data points represent a range in muzzle velocity of more than 1,250 fps within a single group (e.g., the 0.220 group), yet deflection is relatively constant. Over- all, for these cartridges/bullets, ballistic coefficient is the more significant factor governing wind deflection. If desired, plots at 100-yard intervals can be constructed
to determine the suitability of the cartridges/bullets for shots at varmints out to 500 or 600 yards. What is more revealing, however, is to develop plots of wind deflection as a function of distance for the cartridges/bullets in each of the five groups of average ballistic coefficients (Figure 2, above). Examination of this second figure allows an immediate comparative visual assessment of the relative merits of varmint cartridges/bullets with low- and high-value ballistic coefficients, independent of any separate consideration of velocity. Again, it is evident that a major reduction in the deflection
Page 64 Spring 2013
considerable attention, varmint cartridges with lead-free com- posite bullets are clearly at a disadvantage in windy conditions. For example, the 0.155 curve (based on ballistic coefficients for .224 lead-free composite bullets) shows that with crosswind speeds of 10 mph and higher, wind deflection becomes excessive at distances beyond 200 yards. Furthermore, these bullets lose their velocity and striking energy quickly, reducing their abil- ity to anchor varmints at longer distances. If shots beyond 250 yards will be required and windy conditions are contemplated, cartridges with bullets having a ballistic coefficient greater than 0.220 will ensure a better chance for accurate shot placement and of retaining sufficient energy to anchor the critter on the spot. In addition to using cartridges with bullets having as high
a ballistic coefficient as possible, a varmint hunter can reduce the deflection guesswork on longer shots by: 1) employing a spotter and locating the shooting station so that the wind direction is approximately from 12:00 or 6:00; 2) using a scope with a ballistic compensation reticle that includes wind-deflection graduations; or 3) making click adjustments to compensate for average wind conditions. However, the latter option can be risky when mul- tiple shots with different adjustments are required. It is easy to lose track of adjustments in a field environment. A final thought: No matter how fast the muzzle velocity of a varmint bullet, the ballistic coefficient is a critical factor for making successful shots in windy conditions at distances beyond 250 to 300 yards.
REFERENCES Forker, Bob, 2010. Ammo & Ballistics 4, Long Beach, CA,
Safari Press Inc., 438 p. Litz, Bryan, 2009. Applied Ballistics for Long Range Shooting,
Cedar Springs, MI, Applied Ballistics, LLC, 536 p. Rinker, Robert A., 2003. Understanding Firearm Ballistics,
Fourth Edition, Mulberry House Publishing, 427 p. Sierra Bullets, 2003. Rifle and Handgun Reloading Data, Fifth
Edition, Sedalia, MO, 1150 p.
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