The remaining task is to calculate the drift for the Wind Late scenario. From the previous discussion, we know that drift from the first wind segment is zero because the wind velocity is zero for the first 500 yards of flight. The drift from 500-1000 yards, and therefore the total drift, can be calculated by:
Now that our calculations are complete, what conclusions can be drawn? First, as it relates to drift experienced at the target, it’s apparent that bullets are much more sensitive to wind near the muzzle than they are to the same wind near the target. Actually, for this particular set of circumstances, the early wind caused approximately twice the drift of an equivalent late wind. Second, when the drift seen in the Wind Early scenario is added to that observed in the Wind Late scenario, the result (68.73 inches) corresponds exactly to the independent calculation we performed for a constant 10-mph crosswind acting over the entire 1,000-yard range. Since the net effect of combining the Wind Early and Wind Late cases results in a constant 10-mph crosswind across the full 1,000-yard range, logic dictates that total bullet drift be the same also.
The key finding is that approximately two-thirds (45.85/68.73) of the total drift seen at 1,000 yards for a constant 10-mph crosswind is attributable to the first 500 yards of bullet flight, while only about one-third is caused by the second half of flight. This has practical implications for the long-range shooter. Most of us carry a data card or some other means of referencing important ballistic data, so we can make corrections for range, shooting angle and wind. Usually, drift data for a constant 10-mph crosswind is referenced. When faced with a variable wind the aforementioned information can help us make the proper adjustment.
As an example, consider the following hypothetical situation: While hunting mule deer you glass a nice buck, which you subsequently range at 800 yards. For a shot of this length in the presence of a constant 10-mph crosswind, your data card predicts 41 inches of drift. As you evaluate the wind, however, you’re confident you’re dealing with a 10-mph crosswind at your shooting position but you’re just as sure there’s little wind near the target. If you feel the wind is steady until about halfway to the buck (400 yards), then you can be comfortable correcting for two-thirds of the drift listed on the data card, which amounts to 27 inches (41 x ⅔ = 27.33). If you determine that the wind is a factor for only the first 300 yards, using 50% of the listed drift (20.5 inches) would be a good approximation; if you feel the wind maintains its gusto to the 500-yard mark, then correcting for 75% of the 800-yard drift (31 inches) might be appropriate.
This presentation provides a true scientific rationale for correcting for bullet drift in variable crosswinds. I’ve demonstrated the relevant mathematical formulas and calculations as a means of proving the validity of my statements. However, an appreciation of the practical effects of wind on a bullet is much more important than being able to manipulate formulas. Towards that end, the only thing that needs to be remembered is the ⅔-⅓ rule. That is: in the presence of a constant crosswind, approximately two-thirds of the drift seen at the target is caused during the first half of bullet flight, while the remaining one-third of the total drift results from the second half of the bullet’s journey.
Armed with this information, the long-range practitioner can make more accurate corrections in shooting situations where variable crosswinds are present. Version 6 of the Sierra Infinity ballistic software has the ability to calculate total bullet drift for scenarios consisting of a maximum of five separate wind segments. This capability can be used to sharpen one’s wind-compensation skills. Hypothetical wind problems, featuring wind segments of varying lengths, wind speeds and directions, can be evaluated using one’s data card, common sense and the information discussed here. Once a prediction of total bullet drift at the target has been estimated, it can be compared to the precise formulation produced by the software program. Over time, these exercises will pay dividends when a shot really counts.
References: McCoy, Robert L., Modern Exterior Ballistics, Schiffer Publishing Ltd.,
Atglen, Pennsylvania, 1999
©Copyright 2011 by Paul C. Carter
Paul C. Carter is a big-game hunter and author who resides in Massachusetts. He has hunted game across North America, taking numerous animals, many with a muzzle-loading rifle. He especially enjoys hunting sheep and whitetail deer. Paul has two Grand Slams of North American wild sheep to his credit, one of which was taken with his iron-sighted muzzleloader. He’s also written two books: Tracking Whitetails: Answers to Your Questions and Great Shot! A Guide to Acquiring Shooting Skills for Big-Game Hunters.
Join the discussion of this article HERE at the Article Discussion Forum.
<Previous | Home