Understanding Bullet Drop
Understanding Bullet Drop
In hunting, or when firing at an enemy over an unknown distance, the importance of a flat trajectory is evidentóbut it is still important even where the distance is definitely known. When firing at long range a delicate estimation of distance is necessary to obtain accuracy of fire, even when using the best and most accurately graduated sights. Any ordinary man can be drilled to estimate distances up to 600 yards with great accuracy and dispatch. Distances may be appreciated by the eye alone or by the aid of instruments. The latter method is of no practical value in the field before an. enemy, and should only be employed on the drill ground as an aid. The writer's stadiometer the principle of which is based on the proportionality of the corresponding sides of similar triangles and an application of the plummets, does good work on an undulating and broken drill ground, where actual chain measurement would be impracticable if not impossible. Prolonged practice and experience in the appreciation of distances are necessary to give the coup d'ocil that insures sufficient accuracy. The practice should be conducted over smooth, broken and undulating ground, and frequently from elevated points. The distances should also be estimated in all directions as regards light and the condition of the atmosphere.
The following are the important causes which vary the direction and intensity of the forces acting on the bullet, and which may be greatly obviated by carefulness and an understanding of the subject:
1. A frequent cause of inaccuracy of fire is a false or defective barrel, short swells and long depressions being often found on its interior. These swells or ridges, by increasing the friction, may so effect the recoil as to have an injurious effect on the fire, or so affect the exterior form of the bullet as to produce an irregularity in its motion. The depressions or swells change the interior lines of the piece and give the bullet a false direction.
2. Another cause of inaccuracy of fire is the vibration of the barrel when firing, caused by the want of a homogeneous distribution of metal about its axis, and often to binding bands. These vibrations tend to alter the direction of the bullet as it leaves the muzzle.
3. Recoil causes the man to turn to the side from which he fires, and produces deviation in that direction. It is supported by pressing the butt firmly against the shoulder with the right hand, the left hand supporting the weight of the rifle, and varies with the position of the rifle relative to the horizontal, being a maximum when the shot is fired vertically upward. The shock of the recoil against the shoulder is diminished by the bend in the stock, serving to decompose the force into two components, one acting through the stock against the shoulder the other in the direction of the axis of the barrel, tending to raise it. Whatever lessens the recoil theoretically increases the range.
4. When the bullet reaches the muzzle of the rifle, it will revolve about its axis nearly 800 times in a second, and a point on its exterior side surface will have an axial velocity of about ninety feet per second. This in connection with the resistance of the air produces a lateral drifting of the bullet in the direction in which the grooves have a turn. This is known as drift, and is greater in the descending than in the ascending branch of the trajectory. It increases as the diameter of the bullet, the angle of fire, the velocity of rotation and the range increase, and as the velocity of translation decreases. The drift in the Springfield rifle (caliber 45, seventy grains powder) at 500 yards is two feet.
5. The pull of the trigger should not be too great, a three-pound pull being the minimum. It should be pressed by a steadily increasing pressure of the finger in the direction of the axis of the barrel, without communicating motion to the rifle, the breath being held until the hammer falls. If the trigger is too hard and is pulled convulsively, the muzzle will be turned to the right. There should be a quick and decided connection between the mind and finger. Few men can pull off the trigger of our average service rifle with the first joint of a single finger. 1 have frequently supported the whole weight of the rifle, at full cock, on the trigger, without pulling it off, and I consider this a very serious defect. Every man should invariably fire his own and the same gun, in order to become acquainted with its defects of construction, and learn how to make allowances for consequent deviations from the theoretical trajectory.
6. The principal cause of the inaccuracy of most rifles is that they are sighted too coarsely. Without apparent movement, the rifle may be sighted on any object within a horizontal radius of many feet. I think it best to replace the ordinary bead sight by the Beach Combination Sight, which is such that it forms either an open bead or a globe sight with cover, according as the leaf is turned up or down, thus adapting it either to hunting or target use (and if I mistake not our frontier field service is more on the order of hunting than range practice). The globe of this sight is so constructed as to permit the use of all descriptions of sights, detachable pieces of the various forms in use being slipped into a slot in the globe and held by a screw.
The Lyman sight is a most excellent one when the target is a moving object. When aiming, it has the appearance of a ring or hoop which shows the front sight and the object aimed at, without intercepting any part of the view. Its rim may be instantly changed to give it a large aperture with a narrow rim, or a small aperture with a wider rim. For all quick shooting the large aperture should be used. It possesses the following advantages: it allows an instantaneous aim to be takenóthe object being sighted as quickly as if only the front sight were used; it readily permits one to shoot moving objects, running or flying, with both the eyes in use; it is also very accurate, simple and strong. Any kind of front sight may be used with it, and it may be put on any rifle in the same way that a peep sight is attached and adjusted for shooting any distance up to 1,000 yards.
7. It will be readily seen that a defective position of the line of sight will cause an inaccuracy of fire, and this may be occasioned by a false position being given to either the front or rear sight. If the front sight be to the right of its proper place, the bullet will go to the left and vice versa. The bullet will also be raised (range increased), since the top of the sight is lower than it is when in its proper position. If the rear sight be to the right or left of its true place, the bullet will go to the right or left, and will be lowered (range diminished), since the top of the sight will be lower than when in its true position.
8. A very frequent cause of inaccuracy of fire is the incorrect graduation of the rear sight. Of course, if the elevations corresponding to certain ranges are not accurately marked, the fire will be wild. In determining the graduation for any particular rifle, avoid all proportions and make a series of experiments with the greatest care. With a properly made arm and cartridge, and the elevating sight accurately graduated, anyone can, by care and practice, become a good marksman.
9. A serious cause of inaccuracy, originating with the firer, is the faulty position that he gives to the musket in firing, by inclining to the right or left, which tends to carry the bullet to the side to which the rifle is inclined, and to diminish the range. When firing at long ranges a trifling inclination to the right or left will throw the bullet very wide of the target.
10. To prevent inaccuracies while aiming, in addition to keeping the sight vertical, the firer should observe the following: The eye should glance from the sights to the target, being constantly on the target. If the particular rifle carries higher or lower than the average, it must be remedied by aiming with a fine or a coarse sight. In aiming raise the rifle. Upward motion acts against gravity, and has a tendency to prevent any lateral motion of the muzzle. Hold the butt firmly against the shoulder, and do not turn the head away at the instant of pulling the trigger. Fire low rather than too high.
11. After firing a few shots on a dry, hot day, the bullets gradually fall lower and lower, in consequence of the fouling of the barrel. The barrel should be kept clean and, as far as possible, not over-heated.
12. The condition of the atmosphere noticeably affects the course of the bullet. The more moisture there is in the air, the less elevation required. The bullet is frequently noticed to fall immediately after a rain. Warm air offers less resistance to the bullet than does cold air. A fall of 20 degrees in temperature will cause the bullet to lower ten to eleven inches at 300 yards range. In firing over water the elevation must be increased, in consequence of the lower temperature of the air over the water. In ascending the mountain the air becomes more and more rare, and consequently the resistance to the bullet is less on the mountain than at its base. Mirage, an optical illusion occurring in level districts on very warm days, causes the target to apparently rise in the air and become distorted in shape. This materially affects such objects as are near the ground, and engenders a tendency to shoot too high.
13. The influence of light and shade on the firing is very remarkable. On a bright day the target is refracted so as to apparently stand higher, which would theoretically require a lower elevation than on a very dull day. When the light shines directly on the target, when the target is against a light background (so that the details are better brought out), when the sun shines on the firer's back, when the atmosphere is clear, when the ground is level and uniform in appearance or when it gradually rises toward the target, the same will appear much nearer, and will theoretically require a higher elevation. The best shooting is invariably done on cloudy days when the sun's light is evenly diffused. It is very difficult to shoot well when passing clouds intercept portions of the sun's light and heat. It is readily seen how this disturbance might set up currents in the air which would tend to carry the bullet from its course, and how the rays of light deflected from their course before reaching the eye would cause the target to apparently occupy a false position. It will be well to diminish the elevation should the sun suddenly appear and light up the target while the firer still remains in the shade, and to Increase it should the target remain in the shade while the sun shines on the firer.
14. Bright sights and barrels are obviously objectionable. The reflection of the sun's light on the sights causes them to appear as brilliant points and precludes the possibility of an accurate aim. If the sun's rays come laterally, the trouble will be yet greater, inasmuch as they will brighten the rear side of the front sight and the opposite side of the rear sight notch, and cause a tendency to shoot away from the sun. The refraction of the sun's rays from the polished barrel causes the target to become indistinct and to assume the appearance of motion. The sights and barrel about the muzzle should be blackened with smoke if nothing better is at hand.
15. The effect of the wind upon the trajectory and the allowance to be made therefor are most troublesome questions for the marksman. Inasmuch as the wind is continually changing in intensity and direction, it is almost impossible to make tables of allowances for it. The best skill and judgment of the marksman are brought into play when firing in mountain districts, where there are many cross-currents with which to contend. All winds, except toward the target, retard the bullet and render a higher elevation necessary. A wind from the rear helps the bullet and tends to high shooting. Experience has shown it necessary to alter the wind-gauge twelve or more feet between two consecutive shots over a range of 1,000 yards, in order to make a bullís-eye each time when the wind was too high or variable. The inclination is generally to under-estimate for wind allowance, nearly every one disliking to aim far away from the target.
Farrow, Edward S. American Small Arms; a Veritable Encyclopedia of Knowledge for Sportsmen and Military Men. New York: Bradford, 1904. Print.
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