Ballistic Coeffi cients And The Varmint Hunter Roy Welch
the small kill zone of ground squirrels, prairie dogs, woodchucks, foxes, and coy- otes at distances of less than 100 to more than 400 yards. This is possible, provided the shooter has an accurate rifle/cartridge combination, is able to measure the dis- tance to the target, has a stable shooting position, and knows the exterior ballistics for the cartridge. In particular, the effect of the wind along the bullet path and the amount of hold off or sight adjustment necessary to ensure a killing shot must be determined. Wind speed/direction, muzzle
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velocity, and the ballistic coefficient of the bullet determine the amount of wind deflection. Most hunters put a premium on velocity, but tend to ignore ballistic co- efficient. When using a centerfire varmint rifle in light wind conditions at distances out to 200 to 250 yards, the ballistic coeffi- cient is not a serious issue. However, when shots must be taken at longer distances and the wind is blowing at more than 5 to 10 miles per hour, failure to consider the bullet’s ballistic coefficient can lead to missed targets and shooter frustration. The ballistic coefficients of com-
mercially available varmint bullets range from approximately 0.130 to about 0.470, with lower values representative of bul- lets that lose velocity (and energy) quickly. Such bullets are very susceptible to wind deflection. Bullets with higher values are less susceptible to wind deflection and ex- tend the range at which shots can be taken with a better than average chance of suc- cess. The mathematical basis for ballistic coefficients is presented in Rinker (2003), Sierra Bullets (2003), and Litz (2009), but is beyond the scope of this brief article. Instead, the focus here is on the ballistic coefficients associated with groups of bullets employed in popular varmint cartridges and the reduction in wind de- flection possible by using cartridges with bullets having higher ballistic coefficients. In the following discussion, ballistic
coefficients and 10-mph crosswind deflec- tion values were obtained for selected centerfire varmint cartridges from Bob Forker’s Ammo & Ballistics 4 (2010), avail- able from Safari Press, and from manufac-
armint hunters must be able to consistently place shots within
turers’ data. Using these source materials, 32 varmint cartridges of 17 to 25 caliber with spitzer bullets from Barnes, Horna- dy, Nosler, Remington, Sierra, Speer, and Winchester were placed in five groups corresponding to the following average ballistic coefficient values: 1) 0.155, typical of the ballistic coefficients for 0.224 lead- free composite varmint bullets; 2) 0.220, representative of the ballistic coefficients for most 0.172 to 0.224 lead-core varmint bullets; 3) 0.275, the approximate ballistic coefficient for 40-grain 0.204, 55-grain 0.224, and 55- to 65-grain 0.243 bullets; 4) 0.360, characteristic of the ballistic coef- ficients for 68- and 69-grain 0.224, 85- to 90-grain 0.243, and 100-grain 0.257 bul- lets; and 5) 0.420, the approximate ballistic coefficient for 75-grain 0.224 and 95- and 100-grain 0.243 bullets. Muzzle velocities for the various cartridges/bullets ranged from approximately 3,000 to 4,250 feet per second. A plot of the relationship between
wind deflection at 300 yards in a 10-mph crosswind and ballistic coefficients re- veals the five groups that correspond to the above averages (Figure 1 below). In addition, this figure provides a graphic in-
sight to the advantages of higher ballistic coefficients, particularly when “Kentucky windage” is required to compensate for deflection in order to ensure a killing shot on small varmints. Kentucky windage hold-off estimates are typically made in relation to the body width of the targeted varmint. For the purposes of this discus- sion, standing ground squirrels, prairie dogs, and woodchucks/rockchucks are assumed to be the targets of interest with widths of approximately 2, 3, and 4 inches, respectively. Field experience has shown that windage hold-offs up to an equivalent of three varmint widths can be estimated to a reasonable degree of reliability. Thus, for these varmints, 6, 9, and 12 inches are about the borderline maximum hold-offs beyond which the chances of a hit become more guesswork than science. In Figure 1 the deflection equivalent
to the 3X width of a prairie dog is rep- resented by a horizontal line extending from the 9-inch deflection value. This line passes below or above the groups of circled data points. All the cartridges/ bullets with ballistic coefficients of less than the 0.275 average have deflection val-
Wind deflection values for 32 selected varmint cartridges/bullets fall into five groups with average ballistic coefficients of 0.155, 0.220, 0.275, 0.360, and 0.420. A horizontal line extending from the 10-mph crosswind deflection value of 9 inches (the 3X width of a prairie dog) at 300 yards indicates that cartridges with bullets having ballistic coefficients of less than approximately 0.275 are not well-suited for long-range shots at prairie dogs in windy conditions.
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