Range and Windage Estimates
Range and Wind Estimation
Range and wind are the variables that will have the most effect on your bullet's trajectory. Fortunately, if they are quantified correctly, they are also the easiest to account for. Altitude (air pressure) will have a lesser effect and can be determined ahead of time by simply looking at a topographical map of your area of operations. Gross changes in altitude will usually require establishing new trajectory tables by either shooting at the new altitude or running a ballistics program set up for the new altitude. Either one can be done well ahead of time. Temperature and humidity aren't consistent in their effects on different rifles and loads. You must establish parameters for your rifle by firing under different conditions to determine exactly how your rifle and load will react to changes in temperature and humidity. Slope also has a relatively small effect but it is easily adjusted for. Moving target leads can be a bit tricky but they are a consistent relationship between bullet time of flight and the targets speed lateral to the shooter. The effects of lighting are visual and subjective for each individual shooter.
The various range estimation techniques differ in accuracy. Some are faster than others to apply and some require specialized equipment. Different techniques will be suitable depending on the situation. This is a list of the techniques in approximate order from most accurate to potentially least accurate. I prefer to figure my ranges in yards as it relates better to minute-of-angle (MOA) measurements and makes the mil-dot formula easier to apply to human targets.
Physical measurement of the distance– This may be possible when establishing a defensive perimeter or in previously occupied positions.
Pace counts– In a retrograde or delaying action it may be possible to measure range with a pace count. You must know how long your paces are over different types of terrain.
Maps or scaled aerial photos– You must accurately identify your position and that of the target. GPS readings may work quite nicely for this.
Sighter shots– If you can take a sighter shot, you can determine your exact drop. You may be able to take a sighter shot when your shot is masked by other fire or if you are in a secure position. A sighter might be taken from a semi-permanent hide before targets appear in an area. A missed shot should be used to make corrections if another shot is possible.
Range cards– These are only as accurate as the method used to prepare them. An accurate range card is quick and easy to use but takes time to prepare.
Laser rangefinders– Within their limits, lasers are fast and accurate. The new ones are relatively small and light as well. Lasers require batteries. They don't function well in poor visibility or with ill defined targets.
Stereoscopic range finders– These don't suffer some of the functional limits of the lasers but tend to be bulkier and more difficult to use accurately. Stereoscopic range finders must be correctly calibrated to give accurate readings. The small civilian units made by Ranging can be as accurate as a mil-dot scale. The large Barr and Stroud and similar units are extraordinarily accurate but they are very large and no longer manufactured.
Mil-dot reticles– If the mil-dot is part of your rifle scope it doesn't require an extra piece of equipment. Mil-dots are fairly fast to use and can be very accurate. Other types of ranging reticle are also available.
Man made reference points– City blocks, evenly spaced power poles or fence posts can be counted to give fairly accurate distances if the spacing is known and regular. Man made artifacts of known height are also useful for getting readings with a ranging reticle. Examples include power poles, doorways, or vehicles of known height.
Ranging reticles in variable power scopes– I dislike the increased complexity of a variable scope for military purposes. Additionally, in order for a variable scope to work as a range finder, the ranging portion of the reticle must be in the rear focal plane. This can cause shifts in position relative to the point of impact with changes in magnification. The Leatherwood ART, Burris ARC, Leupold VariXIII, and Redfield Accutrak are some examples of variable ranging scopes.
Comparing the size of a man against a duplex reticle– The portion of a man that a standard fixed duplex reticle covers can give an idea as to range.
Estimating by eye– This is the least accurate method but also the fastest and requires no equipment. It does require a great deal of constant practice and changes in altitude, humidity, or terrain can confuse even the most experienced. Know your pace length. Spot an object and estimate the range. Pace it off. Pick a new object at an angle to your original path and do it again. Continuous practice will enable you to estimate ranges with fair accuracy.
Mentally break the distance into 100 yard increments– By picturing the number of football fields between you and the target you can improve your eyeball estimate.
Estimate half the distance– It may be more accurate to pick a point halfway to the target and estimate the range to that. By doubling that estimate you'll have the full range.
Bracket your estimates– Determine the distances at which the target can be no-closer-than and no-farther-than and split the difference. The closer you can bracket the range, the more accurate your estimate will be.
Average the sniper and spotter estimates– The average of both team members' estimates will probably be more accurate than a single estimate.
Gauge the appearance of objects– How big an object looks or how much detail can be made out can give you an idea of the range with experience. This is an extremely subjective technique and changes in air clarity will affect it greatly.
The old Rule-of-Thumb method is accurate out to a couple of hundred yards but modern cartridges don't require much holdover at that range.
Unless you or your target are moving, range is won't change significantly before you get a chance to shoot. Wind, on the other hand, is very unlikely to remain constant for any length of time. You must accurately estimate both speed and direction. You must also accurately predict what terrain effects on the wind will be between you and the target. It's possible to have to deal with more then one wind direction and speed at any given time over the path of the bullet. Generally the wind closest to the shooter has the most effect. I figure my wind speeds in miles per hour as that is what my formulae are geared to.
Wind gauges or anemometers– These will give you a precise reading of wind speed. Unfortunately, unless you can place anemometers at several points along the bullet path and at the height above the ground at which the bullet will travel, you will only get a partial picture of the wind patterns. Weather reports can give you a rough idea of conditions in your area.
Observation of physical effects– Accurate observations of the physical effects of wind between you and your target can give you a good overall picture of the wind patterns and a fairly accurate estimate of the wind speeds.
Estimating the angle of smoke, flags, or a dropped handkerchief– This is an accurate method up to about 23 miles per hour.
The appearance of mirage– Mirage can be used to estimate wind velocity and direction, as well as humidity and temperature. Judging mirage takes lots of experience under varied conditions. A simple estimate of the mirage angle is good for wind speeds up to ten or fifteen miles per hour. Extremely experienced shooters can estimate wind speeds up to thirty miles per hour. Mirage can also play havoc with your sight picture at long ranges.
The effects of temperature on trajectory are easy to calculate. The effects of temperature on your barrel and the powder in the cartridge, and the resultant changes in muzzle velocity, aren't so easy to calculate. Generally, as the temperature increases so will your bullet strike. Temperature up, sights down. You will have to determine by how much.
Water vapor is actually less dense than dry air. High humidity decreases the air resistance acting on your bullet and will cause it to strike high. Humidity up, sights up. The effects, however, will be very small. The greater effect will be to visibility as increased humidity means more mirage and the air will appear hazy. Dry air is easier to see through.
As altitude increases, air density decreases and your shots will strike higher. Altitude up, sights up. A ballistics program can tell you how much. An increase in altitude also means an increase in visibility. Objects at high altitudes will appear closer than they do at lower altitudes. Compared to range and wind all these variables have relatively minor impact on your trajectory.
Slope is easy to measure and the effects are constant. Whether shooting up or down hill your shot will strike high. It will never strike higher than the total drop of your bullet from the horizontal boreline. You can calculate your new drop by either modifying the range or by modifying the drop. The table the SEALs work from modifies the range. The table you'll find in most reloading manuals modifies your drop. I prefer the SEAL method but either one works fine.
The effects of lighting direction are purely subjective. By paying careful attention to your sight picture and keeping the reticle centered in the scope, you may notice no change in impact at all. Other people may find their bullet strike moves in the direction of the light source. Light up, sights up. Light left, sights left. Only you can tell by how much, if at all.
To accurately estimate and adjust for all the variables it is necessary to shoot under as many different conditions as possible. A chronograph and ballistics program a great to establish a baseline. Keep a data book and record all the conditions as they apply to each and every shot you take. Measure the variables on the range and develop a set of trajectory tables for your rifle and load. As you gain experience your estimates will gain in accuracy. If you have time in the field work directly from your trajectory tables. Formulae are handy if, for whatever reason, you don't have access to your tables. Kentucky windage and elevation are very fast but the least accurate and– while every shooter should be skilled at holding off– should be considered a last resort.