A Brief Course in Ballistics

As a long range rifleman, you should read this material carefully.  William Davis, a master of ballistics, explains in simple English a concise, informative explanation on how real time key factors influence bullet performance.  After reading this section, you will understand why Aiming Solution software, using the ATrag program is a must for extended range shooting.

Effect of Barometric Pressure on the Bullet’s Flight

Barometric pressure affects the down-range performance of a bullet because it affects the density of the air through which the bullet must travel.  The barometric pressure depends primarily on altitude and to a much smaller extent on the constantly occurring changes in the atmosphere that produces the barometric “highs” and “lows” that we hear about in weather reports.

The barometric pressures given in weather reports are not actual pressures; they have been adjusted to eliminate the effect of altitude.  The adjusted pressures given in weather reports everywhere are typically between 29 and 31 inches of mercury, although the actual barometric pressure in Denver (which is above 5000 feet) is typically about five inches less than the pressure in a coastal city such as New York.

Adjusted pressures are more suitable than actual pressures for weather forecasting, and users of barometers are generally advised to adjust their barometers to the pressure being reported in a local radio or television broadcast.  This adjustment makes their readings comparable to the readings of other barometers for weather forecasting, but the readings are then unsuitable for use in determining the atmospheric density.

The shooter who wishes to use a barometer for determining atmospheric density should take it to a facility where an accurate barometric reading is available, such as an airport control tower or a science laboratory at a college university.  He should explain clearly that the information he requests is the current actual barometric pressure and not the pressure after the adjustment for altitude has been applied.  He should adjust his instrument on the spot to the actual pressure, and henceforth the readings obtained from that instrument will reflect not only the effect of altitude, but also the effect of weather- related fluctuations in pressure, wherever the instrument may be.  It is this actual barometric pressure that is called for when the user elects to input the barometric pressure explicitly in the computer program that accompanies the HORUS sighting system.

Of course the user of the HORUS computer program may alternatively choose to input the altitude (as obtained from a topographic map, for example) rather than to input the barometric pressure explicitly.  In that case, the program will calculate the air density, with only slightly less accuracy, based on the inputs of altitude and temperature in which the barometric pressure is implicit.

Effect of Air Temperature on the Bullet’s Flight

The temperature of the air affects the aerodynamic drag (“air resistance”) encountered by the bullet in its flight.  There are two reasons for this.  The first and more important reason is that cold air is denser than warmer air, under conditions that are otherwise the same.  The second reason is that the speed of sound is lower in cold air than it is in warmer air.

The first reason is the more easily understood.  Most people probably know that almost every substance contracts and becomes denser as it is cooled, and therefore that cold air is denser than warmer air.  They also know that water is much denser than air, and almost everyone has noticed that much more effort is required to wade through deep water than would be required to walk along at the same speed surrounded only by the air.  The denser the medium through which a body passes, the more energy is expended (and the more velocity is lost) by the body as it passes through.

The second reason is not intuitively obvious, but it is a fact that cold air offers greater resistance to the passage of a bullet- especially a supersonic bullet- because the speed of sound is lower in cold air than it is in warmer air.  A full explanation of this phenomenon would be too lengthy to include here, but it is a fact that the force of aerodynamic drag on a fast-moving body depends upon the so-called “Mach ratio” which is the ratio of the speed of the moving body to the speed of sound.  Because the speed of sound is lower in cold air, the Mach ratio for a bullet at any particular velocity is correspondingly higher in cold air, and the force of aerodynamic drag is therefore greater.

Effect of Relative Humidity (RH) on the Bullet’s Flight

The relative humidity affects the down-range performance of a bullet because it affects the density of the air through which the bullet flies. Contrary to what many people suppose, humid air is less dense than dry air under the same conditions of temperature and barometric pressure, because the molecular weight of water is less than the molecular weights of the principal gases (nitrogen and oxygen) that comprise our atmosphere.

The effect of humidity on the down-range performance of bullets is small in comparison to other factors that affect the downrange performance.  The relative humidity has greater effect on air density at high temperature than at low temperature, but even at 90 degrees F.  The difference in density between completely dry air and completely saturated air is only about 1/10th of one percent.  For a typical .30-caliber 150-grain bullet having a ballistic coefficient of C1=.400 and a muzzle velocity of 2800 FPS, the difference in remaining velocity at 1000 yards is about 14 FPS, and the difference in drop is about six inches.

Effect of the Earth’s Rotation on the Bullet’s Flight

The Coriolis effect on the path of a projectile is a consequence of the rotation of the earth, and the fact that the surface of the earth is curved (spherical) rather than flat as we generally assume it to be for the solution of problems in exterior ballistics.  The magnitude and direction of the Coriolis effect depends on the location of the gun (its latitude) and the horizontal direction (azimuth) in which the gun is pointed.  The Coriolis effect is so small in comparison to other effects on the projectile’s path that it is not ordinarily considered except in the case of long-range artillery fire, but of course it does actually have some effect on all projectiles.

A somewhat simplified way to visualize the problem is to consider that, because of the earth’s rotation, a “stationary” target is not truly motionless as we normally assume it to be, but is constantly moving.  Consequently, the point on the target at which the projectile was directed when it left the muzzle will have moved some small distance (relative to the gun) during the time the projectile is in flight.  In this sense, the correction for the Coriolis effect is similar to the “lead” required to hit a moving target.

The Coriolis effect on projectiles fired either due north or due south is entirely horizontal, and the Coriolis effect (on a vertical target) on projectiles fired either due east or due west is entirely vertical.  The effect on projectiles fired in any other direction is both horizontal and vertical, the amount of each depending upon the horizontal direction (azimuth) in which the gun was pointed.

Effect of Wind on the Bullet’s Flight

The most important effect of wind on the bullet’s flight is to change its direction horizontally.  In the language of gunnery, angles in the horizontal plane are called angles of deflection, and so the effect of a cross-wind on the bullet’s path is correctly called wind deflection effect although the term “wind drift” is often used, rather loosely, instead.

To hit targets at long range, riflemen must learn to estimate the cross-component of the wind velocity.  The speed and direction of the wind can be measured by suitable instruments, or estimated by experienced observers from signs such as the motion of leaves and grass, and the appearance of “mirage” which is the wavy pattern of distortion that is seen through a powerful telescope, caused by refraction of light passing through waves of heated air as they rise from the ground.  Wind flags and other indicators are placed downrange during some types of rifle competition to aid in estimating wind effects.

Wind deflection depends upon the cross-component of the wind velocity.  A 10-MPH wind blowing from 3 o’clock or 9 o’clock has a cross-component of 10 mph.  10-MPH winds from 2, 4, 8 or 10 o’clock have a cross-component of about 8.7 MPH, while winds from 1, 5, 7 o’clock have a cross-component of 5.0 MPH.  Winds blowing from 6 or 12 o’clock have no cross-component.

Wind speed and direction are practically never uniform over the whole distance from the gun to the target, and so the rifleman estimating the allowance for wind must decide whether to concentrate his attention on the wind nearer the gun or on that nearer the target.  The answer is that the wind conditions near the gun have much greater effect than the conditions near the target.  Two hypothetical examples will illustrate this point.  For both examples, assume that the rifleman fires a 150-grain .30-caliber bullet having a ballistic coefficient of C1=.44 at a muzzle velocity of 2800 FPS, toward a target 500 yards away.

For the first example, suppose that a perfectly uniform 10-MPH wind blows from 9 o’clock across the first 100 yards of range, and that there is no wind whatsoever over the remainder of the range from 100 yards to 500 yards.  Consider now the situation when the bullet has reached 100 yards.  We see that the wind deflection is about 0.8 inch, which means that the bullet is now 0.8 inch to the right of the line from muzzle to target, but also that its path is curved toward the right at an angle of about 1.6 MOA.  With no further influence from the wind over the remaining 400-yard distance, the bullet’s path in the horizontal plane will be straight, but the horizontal angle of 1.6 MOA that it had already acquired at 100 yards will carry it 6.4 inches farther to the right of the gun-target line, for a total wind-defection effect of 7.2 inches at 500 yards.

For the second example, suppose that conditions from the muzzle to 400 yards are perfectly calm, but the 10-MPH wind from 9 o’clock blows across the range between 400 and 500 yards.  The horizontal direction of the bullet remains directly toward the target in the calm air out to 400 yards, where its remaining velocity is about 1959 fps, when it suddenly encounters the 10-MPH crosswind.  The bullet’s flight between 400 and 500 yards will be the same as that of a bullet of the same kind fired toward a 100-yard target at a muzzle velocity of 1959 FPS, for which we find that the wind deflection would be about 1.3 inches.

Effects of Shooting Uphill or Downhill

The vertical drop of a bullet below its line of departure is practically the same whether the target is uphill, downhill or at the same level as the gun.  That does not imply, however, that the sight adjustment or the allowance in aiming required to hit a target at any range is unaffected by the slope of the gun-target line.  The reason for this apparent contradiction is that the effects of an aiming allowance or an elevation adjustment of the sight are in a plane perpendicular to the line of sight, which, in the case of uphill or downhill firing, is not the same as the vertical plane in which the bullet drop is measured.  The reason we must take account of the slope of the gun-target line is illustrated in the following examples.

Suppose we are firing a .30-caliber 180-grain bullet having a ballistic coefficient of C1=.450 and a muzzle velocity of 2600 FPS, under standard sea-level atmospheric conditions, and that we have sighted-in the rifle at 200 yards.  Suppose further that we wish to shoot at a bull’s-eye on a tall vertical target 700 yards away on the same level as the gun.  We can calculate that, if we were to fire with the sight adjusted for the sight-in range of 200 yards, the bullet would strike about 147 inches low on the vertical target.  Therefore, to hit the bull’s-eye we must either (1) aim 147 inches high or (2) make an elevation adjustment of about 21 MOA (147/7) on our sight.

Now suppose that all the conditions are the same as those described above except that the tall vertical target is on higher ground at an uphill angle of 30 degrees.  Since the vertical drop of the bullet is the same as before, the bullet would strike the vertical target at a point 147 inches below the bull’s-eye if we fired with the sight adjusted for the sight-in range.  However, as we look upward at a 30-degree angle toward the target, the vertical line between the bullet hole and the center of the bull’s-eye would appear to be less than 147 inches long because of the angle from which we are viewing it.  We can calculate by trigonometry that a vertical line 147 inches long would appear to be only about 127 inches (147*cos 30 degrees) when viewed from a location 30 degrees below.  Therefore, we could hit the bull’s-eye by (1) aiming 127 inches high or (2) by making an elevation adjustment of about 18 MOA (127/7) on our sight.

By reasoning similarly, we can see that a 147-inch vertical line would appear to be about 127 inches long when viewed from 30 degrees above the target as well as from 30 degrees below, and therefore the same allowance in elevation must be made in either case.

Effect of Ammunition Temperature on Muzzle Velocity

As almost everyone knows, the muzzle velocity of a bullet is a factor of fundamental importance in determining the bullet’s path.  Therefore, the rifleman must have some knowledge of the expected muzzle velocity in order to decide where to aim or how to adjust his sight.  The best estimate of the expected muzzle velocity is obtained from carefully conducted velocity testing of the ammunition to be used in the field, fired from the rifle to be used in the field, and preferable with the ammunition at approximately the same temperature that it will be in the field.

There are two kinds of factors, random and systematic, that determine the muzzle velocity of any particular shot.  Some degree of random shot-to-shot variation in muzzle velocity is inevitable, and its affect on any particular shot is inherently unpredictable.  The rifleman can minimize the random variation by choosing ammunition that has demonstrated consistently good uniformity in carefully controlled velocity testing.

The ammunition temperature is the principal source of systematic variation in the muzzle velocity, assuming that the ammunition is being fired in the same rifle with which the basic muzzle velocity was established.  Unfortunately, the effect of temperature on the muzzle velocity varies widely from one load to another.  The rifleman minimizes the systematic effect of ammunition temperature on muzzle velocity by measuring the muzzle velocity with the ammunition at approximately the same temperature that it will be in the field.

A Memorandum Report written by Barbara Wagoner of the U.S. Army Ballistic Research Laboratory contains an analysis of a great many velocity tests of ammunition ranging in caliber from 5.56mm to 30mm, loaded with both single-base and double-base powders, and fired at various temperatures from -65 degrees F to +165 degrees F.  The effect of temperature varied quite widely from one load to another, as was undoubtedly expected from previous experience.  If all the various ammunition types are lumped together, however, the data indicate a typical velocity change of approximately 0.4 percent for a change of 10 degrees F in ammunition temperature.  This implies, for example, that a load which produces an average muzzle velocity of 3000 FPS at +70 degrees F would be expected to average approximately 3024 FPS at +90 degrees F and approximately 2976 FPS at +40 degrees F.  These differences are significant if targets are to be engaged at very long range, and that fact underscores the desirability of having reliable muzzle-velocity data for the ammunition at a temperature reasonably close to the temperature at which it will be in the field.

Effects of Ballistic Coefficient on the Bullet’s Flight

The ballistic coefficient of a bullet is the measure of its ability to move through the air with minimum resistance.  This resistance is called aerodynamic drag, and it’s most significant effect is to reduce the velocity of the bullet en route to the target and thereby to increase the bullet’s time of flight.  An increase in time of flight increases the vertical drop of the bullet away from its original line of departure, and therefore it also increases the vertical aiming allowance or sight adjustment required to hit targets at different ranges.  Aerodynamic drag also reduces the striking velocity of the bullet at the target, and it may thereby reduce the bullet’s terminal effectiveness, depending on the nature of the target.

Another important result of aerodynamic drag is that it makes the bullet susceptible to wind deflection, which is the horizontal change in direction of the bullet’s path, caused by wind blowing across the gun-target line.  Contrary to what many people suppose, the effect of cross-wind on the bullet’s path does not depend primarily upon the bullet’s time of flight, but upon the length of time that the bullet is delayed en route to the target by aerodynamic drag.  Any increase in the ballistic coefficient of the bullet tends to reduce this delay time, and it may do so even though the gain in ballistic coefficient is achieved at the expense of a lower muzzle velocity and a longer time of flight.  The following example will illustrate this point.

Consider first a .308 Win load that consists of a 150-grain bullet having a ballistic coefficient of C1=.400, fired at a muzzle velocity of 2850 FPS.  We can calculate that its time of flight to 700 yards, for example, would be about 1.027 seconds, and that its wind deflection in a 10-MPH crosswind would be about 51 inches.

Now compare this .308 Win load to another one using a 180-grain bullet of similar shape, which would have a ballistic coefficient of about C1=.480.  The muzzle velocity attainable with the heavier bullet at comparable chamber pressures would be only about 2600 FPS, and the time of flight to 700 yards would be increased to 1.070 seconds.  Nevertheless, the wind deflection would actually be reduced by about ten percent, from 51 inches to 46 inches at 700 yards, owing to the higher ballistic coefficient of the 180-grain bullet.

The ballistic coefficients of commercial sporting bullets in the U.S.A. are almost invariably based on comparison with the “G1 Standard Projectile” which has a specified diameter and weight, and a particular shape.  A ballistic coefficient based on the G1 projectile shape is properly identified as “C1” to distinguish it from other possible ballistic coefficients such as “C5,” “C6,” “C7”, “C8,” etc. that were once widely used in military sources referring to projectiles of various different shapes.  References to “the” ballistic coefficient (or sometimes “the B.C.”) of a commercial sporting bullet in the U.S.A. should be taken to mean the C1 ballistic coefficient unless the source gives specific notice to the contrary.

Most manufacturers of commercial sporting rifle bullets will provide values of the (C1) ballistic coefficients of their bullets upon request, and several manufacturers include that information in the reloading handbooks that they publish.  One manufacturer, Sierra Bullets, lists several different C1 ballistic coefficients for each bullet, each ballistic coefficient being intended to apply to a different velocity range.  For Sierra bullets fired at a muzzle velocity of at least 2500 FPS, the ballistic coefficient listed by Sierra for a velocity of 2500 FPS will generally produce satisfactory results using the HORUS computer program, over the ranges of practical interest to the rifleman.

Lead

The actual lead required also depends upon some factors that are determined by the actions of the shooter in firing the shot, and therefore cannot be predicted.  These include the particular shooter’s human “reaction time” (time between the instant he decides to pull the trigger and the instant that the trigger is actually pulled), and also the “lock time” (time between sear release and firing-pin impact on the primer) of the particular rifle and the “action time” (time between firing-pin impact on the primer and exit of the bullet from the muzzle) of the load.  The errors introduced by these unpredictable variables are minimized if the shooter uses the “sustained lead” technique of shooting at moving targets, and maintains a consistent “follow through”; the errors are maximized if the shooter attempts to “spot shoot” the target by taking aim at a fixed point ahead of target and then pulling the trigger.

Effect of Drift on the Bullet’s Flight

Drift is one of the phenomena that contribute to the horizontal deviation of a spin-stabilized projectile from the vertical plane containing its line of departure.  Drift is an incidental consequence of gyroscopic precession, which itself is otherwise essential for the satisfactory performance of spin-stabilized projectiles.  It is precession that forces the axis of a spin-stabilized bullet or artillery projectile to change its direction constantly as the trajectory curves downward, so that the projectile flies point-forward with its axis nearly parallel to the direction in which it is moving.  The angle between the axis of a projectile and the direction in which it is moving is called yaw.

After a spin-stabilized projectile has recovered from the effects of certain disturbances that it encountered during launching, it settles into an attitude of relatively small yaw that is called the yaw of repose.  At the yaw of repose, the axis in inclined slightly upward and toward the right for projectiles fired from a barrel having right-hand twist of rifling or upward and toward the left for projectiles fired from a barrel having left-hand twist.

It is the horizontal component of the yaw of repose that causes drift.  For projectile fired from a barrel rifled with right-hand twist, the horizontal component of the yaw of repose is toward the right, which causes the air pressure on the left side of the projectile to be greater than the air pressure on the right side, thereby forcing the projectile to drift toward the right.  The horizontal component of the yaw of repose tends to increase as the trajectory curves more steeply downward, and therefore the drift increases ever more rapidly with increasing range.  The horizontal directions are reversed, of course, for projectiles fired from a barrel rifled with left-hand twist.

The calculation of drift is relatively complicated, and to calculate it very accurately requires detailed information about the projectile that is not available for small-arms projectiles except for a few that have military applications at very long range.  The drift calculations incorporated in the Horus reticle and the corresponding computer program are based on typical values for a class of bullets generally used for long-range rifle fire; specifically, long-pointed boattail bullets of low-drag configuration such as the U.S. military 173-grain 7.62 mm M118 Match and the 650-grain Caliber .50 M33 Ball.  Because the whole contribution of drift to the horizontal deviation of the trajectory is relatively small, the lack of detailed information specific to each of the many different bullets that might be used for long-range rifle fire will not detract seriously from the practical accuracy of the results.

The Elements of Dispersion

In reference to shots fired at a target, dispersion refers to the scattering of the shots around the center of impact.  Small dispersion is synonymous with what is commonly called good accuracy, and large dispersion is synonymous with what is commonly called poor accuracy.  The causes of dispersion are sometimes divided into two classes.  The first, which can be called, aiming error, refers to errors in the direction in which the gun is pointed when it is fired.  The second, which can be called ballistic dispersion, refers to deviations of the bullet from its intended path toward the target after it has left the muzzle.

In the most restricted sense, the term aiming error may refer to the degree of accuracy with which the shooter has aligned the sight on his chosen aiming point at the instant of firing.  In a more general sense, however, the aiming error includes also the shooter’s error in choosing the point at which to aim.  Thus, for example, if the range to the target is much greater than the range at which the rifle has been sighted-in, and the shooter fails to adjust his aiming point or his sight so as to make proper allowance for this difference in range, then the mistake in elevation angle so introduced becomes a part of his total aiming error irrespective of the steadiness with which he aims the rifle.

The ballistic dispersion depends primarily upon the quality of the rifle and the ammunition.  If the shooter can make satisfactorily small shot groups at short ranges such as 100 yards or 100 meters, he has established the quality of the rifle and most of the properties that determine the quality of the ammunition.  The accuracy of the rifle/ammunition system at long ranges cannot be inferred reliably from small groups fired at short ranges, however, because the vertical dispersion at long ranges depends very heavily upon the shot-to-shot variation of muzzle velocity, whereas the short-range accuracy is often quite insensitive to variations in muzzle velocity.

Whereas the ballistic dispersion depends upon the quality of the rifle and ammunition, the aiming error depends upon the skill of the shooter and the capabilities of the sighting system.  It is assumed that serious users of the HORUS sighting system will already have acquired the necessary skills in marksmanship.  The HORUS reticle, and the related computer program, are intended to reduce the total aiming error by helping the shooter to select the correct aiming point (or the correct sight adjustment) based on the conditions under which the shot is to be made.

The information about the prevailing conditions must be provided by the shooter or by his coach or observer.  The various elements of this information are more or less important, depending upon the range to the target and the relative magnitude of each element’s effect on the bullet’s path.  The muzzle velocity, the ballistic coefficient, the range, the wind conditions and the target speed (in case of a moving target) have relatively large effects on the bullet’s path relative to the gun-target line and therefore must be most accurately known.

Moderate differences in the uphill and downhill slope of the gun-target line have only moderate effects on the bullet’s path, and therefore reasonable estimates of the uphill/downhill angle will generally be satisfactory.  The drift (which is caused by the gyroscopic precession of the bullet) and the Coriolis effects (which is caused by the rotation and sphericity of the earth) have relatively trivial effects on the bullet’s path, and they are not often considered in calculation of the relatively flat trajectories that are characteristic of rifle fire.  Nevertheless, provision is made for taking account of all these factors in the HORUS system because all of them make at least some small contribution to the total aiming error which the system seeks to reduce insofar as is practicable.

It must be recognized, however, that neither the aiming error nor the ballistic dispersion can ever be reduced to zero, and therefore whether or not a target will be hit by any particular shot is a question of probabilities, depending on the size of the target, the proficiency of the rifleman, the accuracy of the information about the conditions under which the shot will be made, and the ballistic dispersion of the gun/ammunition system.  The rifleman should learn by experience and recognize realistically the outer limits of the ranges at which various types of targets can be engaged with a reasonable probability of success.

Weather Standards

A. The ATRAG program offers three choices for atmospheric conditions.  The first two choices define “Standard Atmosphere”.

A “Standard Atmosphere” defines the atmospheric conditions for which the “standard” firing tables and other tables giving “standard ballistics” are computed.  In the solution of actual gunnery problems, the corrections for the prevailing atmospheric conditions are made on the basis of the agreed standard atmosphere.

  1. The ARMY STANDARD METRO atmosphere was established at the U.S. Army Aberdeen Proving Ground and was used for many years by the U.S. Army as the atmosphere for which all standard firing tables were computed.  This standard atmosphere was also adopted by the manufacturers of commercial ammunition, and it is still in use by the major manufacturers of commercial ammunition and bullets.  IN the Army Standard Metro, the atmosphere at sea-level has a temperature of 59 degrees F., a barometric pressure of about 29.53 inches of mercury, and a relative humidity of 78 percent.  The atmospheric density under these conditions is about .0751 pounds per cubic foot.
  2.  The “ICAO STANDARD ATMOSPHERE” was defined by the International Civil Aviation Organization during the early 1950’s, and it was adopted in the late 1950’s as the standard atmosphere for all of the U.S. armed forces.  In the ICAO standard atmosphere, the atmosphere at sea-level has a temperature of 59 degrees F., a barometric pressure of about 29.92 inches of mercury, and a relative humidity of zero.  The atmospheric density under these conditions is about .0765 pounds per cubic foot. 

B.  The Horus Vision System offers the rifleman two choices to factor weather data.

  1. Altitude and temperature choice is recommended when computing a range table.  This table is usually attached to the stock of your rifle.

Real time barometric pressure, temperature, and relative humidity choice of field data to help calculate a very accurate firing solution.  Recommended when long range precision is a must.

Is My Variable Power Scope in the 1st or 2nd Plane?

First Focal Plane

If your variable power scopes is in the first focal plane (objective plane), all elevation and windage adjustment clicks are valid regardless of the power setting and point of impact will not change.

Note: Most European and a few American scopes are in the first plane.  A first plane scope can usually be identified by looking through the scope while changing the power.  If the reticle changes size, the scope is in the first plane (objective plane).

Second Focal Plane

If your variable power scope is in the second plane (ocular plane), the values of your elevation and windage adjustment are not valid for all powers of your variable scope.  When shooting at different powers, your point of impact will most likely change.  To find the exact power setting where the calibration values of the adjustment knobs are valid and true, you must read your scope instruction manual, call the manufacturer, check the catalog, etc.  The exact power setting is extremely important.

If your scope has Mil-DOTS, that exact power setting is extremely important when using Mil-DOTS to determine range.  A wrong power setting will yield an incorrect answer for range.

WARNING: When long range shots are to be taken after the proper number of clicks have been dialed in for elevation and windage, you must be sure your vari-power adjustment is set to the correct power.  Failure to use the correct power means your bullet could miss the target.

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