Ii is a well known physical fact that motion cannot be produced except at the expense of time and, therefore, the bullet does not move immediately, as its inertia must first be overcome. Now, possibly the reader has seen either in the moving pictures, or in magazines, extremely slow motion pictures of a golfer driving a golf ball. The ball does not move off the tee immediately when the face of the 265 club comes in contact with it but proceeds to flatten out under the impact in a dough-like manner, shortly moving forward—still in contact with the club—gradually resuming its original spherical form and leaving the club due to its elasticity and the force of die blow imparted to it.
A somewhat similar condition occurs with a flat base bullet. With the passage of time, and the continually increasing gas pressure behind it, the bullet begins to move. But it does not all move at the same time, the base being the first part affectcd and moving independendy of the point. This is r.ot difficult to comprehend if you think of a bullet being fired against a hard surface, in which case chc point of the bullet stops first while the base continues to move, causing more or less flattening of the point end or even complete disintegration of the bullet. A reverse condition takes place upon the initial movement of the bullet forward into the barrel. The base is moved first, and expands, and this degree of expansion is very considerable, even with flat base bullets having stiff jackets. The actual degree of expansion is limited only by the space available within the limits of the barrel and chamber. But finally the point of the bullet also begins to move and the bullet goes forward into the throat of the rifling, whereupon the escape of gas past the bullet is to all intents and purposes checked. The effort of the base to move faster than the point of the bullet may continue while the bullet is traveling as much as two or three inchs or more along the barrel, by which time the entire mass will have obtained the same velocity. This expansion,, of bullet bases is easily proven by sawing off the barrel just ahead of the chamber, firing a cartridge in it and recovering the bullet. The degree of expansion will naturally depend upon the hardness of the bullet and the force that is applied to it.
Barrel Vibrations. The disturbance caused by the 266 blow of the firing pin, the sudden expansion of gasses in the chamber, plus the shock of the bullet moving up against the throat in the barrel, sets up violent vibrations throughout the length of the barrel. These vibrations are divided into two distinct parts, one of which is a whip-like motion of the barrel and the other a true vibration such as occurs, for example, in the string of an instrument when it is picked. These vibrations have a very material effect upon the performance of the arm. If they arc uniform from one shot to another, the rifle will shoot accurately, but if anything is done to disturb their uniformity, which incidentally can be effected by improper stocking, they may not be uniform from one shot to another and there will be a considerable dispersion, no matter how accurate the ammunition itself may be, nor how accurately the barrel may be bored, rifled and chambered.
The vibration of a barrel causes a very considerable angular movement of the muzzle and while this movement might be in any direction across a circlc whose center is the center of the barrel, a barrel will almost always vibrate more or less in a vertical plane. This is natural and logical, becausc there is a certain amount of "droop" in a rifle barrel due to its length and weight. In a long, heavy cannon, the "droop" may amount to as much as a couple of inches, although it is, of course, only a very small amount in a rifle barrel. Nevertheless, when an arm is fired, the first tendency of the vibration is to overcome the "droop" so the barrel moves abrupdy in an upward direction, a movement which is ordinarily referred to as "jump." The angle of departure of the bullet is the angle at which the barrel is elevated, plus the jump, plus or minus any angular movement of the muzzle itself due to vibration, at the instant that the bullet leaves.
Recoil. At the same time the barrel vibrations start, recoil commences, the recoil being the "equal and opposite reaction" to the movement of the bullet. The heavier the bullet or the higher its velocity, the greater the recoil is. In addition to the recoil induced by the movement of the bullet, which is known as the primary recoil, there is a
267 secondary recoil which is caused by the column of gas issuing from the muzzle and coming in contact with the resistance of the atmosphere. As this secondary recoil does not take placc until after the bullet has left the muzzle, it has no effect on the angle of departure. The primary recoil does.
A rifle or pistol is normally supported below the line of recoil, the latter being coincident with the axis of the bore, and the effect of the recoil (other than that of unpleasantness to the shooter) is to cause the arm to rotate or attempt to rotate around the point of support. This causes the barrel to climb and move upward while the bullet is traveling through it and while the barrel is in a state of vibration. The movement of the barrel in recoil should not be confused with its movement due to vibration, when considering problems affecting them, but when shooting it is their combined action that governs the true angle of departure of the bullet.
The greater the recoil, the more the barrel will climb before the bullet gets out, provided that the barrel time is not reduced at the same time. If the velocity of the bullet is increased above normal while ic is passing through the bore, it will leave the muzzle before the barrel has had time to climb as far as it might with a lighter load developing less recoil. But this is getting off the track and the point I wish to make here is that the barrel vibrates regardless of the amount of climb or jump from recoil and plays an important part in the accuracy that is developed on the target by the ammunition.
The general idea of barrel vibration can be seen clearly by means of a simple experiment which anyone can make. While it is not recommcndcd that anyone remove the barrel and action of a rifle from its stock in order to try this experiment, nevertheless, the effect and violence of barrel vibrations can be seen by clamping the receiver of a rifle wkh only the barrel attached to it in a heavy vise bolted to a firm bench. If a 4<U" shape wire is hung over
268 the muzzle of the barrel and the tang of the receiver is struck a moderate blow with a stick or raw-hide mallet, the wire will jump upward for an appreciable distance and may jump off the muzzle of the gun, and this with the receiver clamped tightly in a vise. If a number of these "IT shaped wires are hung at regular intervals along the barrel and the tang is tapped repeatedly with light blows, the wires will move along the barrel and group themselves at the nodes on it. As most barrels are tapered and the nodes are closer together in the heavier parts of the barrel than in the lighter, thinner parts, the separation of the spacers will not be uniform. The illustrations given on Plate XIX show the effects of barrel vibration, one picture showing the spacers arranged along the barrel and the other showing the same barrel after the tang of its receiver had been tapped a number of times without touching the spacers in any way.
The vibrations in the barrel oi any well stocked rifle will be quite uniform from one shot to another as long as the ammunition is uniform, but if the ammunition is not uniform for any reason, whether it be from appreciable errors in the powder charge, variations in the weights of bullets, faulty ignition (whether from poor primers, primers which arc not properly seated, or dirty primer pockets), mixed lots of cartridge cases of varying capacity, or a number of other causes, the angle of departure of the bullets will not be exacdy the same from one shot to another.
The writer was once told that the subject of barrel vibration had nothing to do with handloaded ammunition. Perhaps it hasn't, but the handloading of ammunition has a lot to do with barrel vibration and if the barrel vibration is not reasonably uniform, the results obtained with the ammunition will be unsatisfactory.
Heavy barrels do not vibrate as much as light barrels and the vibrations will be more uniform from one shot to another because of the lesser disturbance of the barrel; the heavy barrel is not as susceptible to small differences that exist from one cartridge to another in the ammunition. 269 It is for this reason that heavy barrels shoot more consist-endy than light ones, rather than due to any special perfection in the rifling or chambering. A take-down rifle will not shoot as consistendy as one with a solid frame, because of the looseness of the attachment between the barrel and the receiver, due to the take-down feature. Even though one of these arms may shoot quite well when new, it may change its center of impact considerably if taken down and put together again, and if frequendy taken down the wear on the interrupted threads on the barrel and in the receiver ring will cause it to become less and less consistent in its shooting or we may say, less and less accurate.
If the bare barrel of a rifle is resting on any solid object, against the side of a tree, for example, when it is fired the arm will be thrown away from the point of support by the vibradons. Therefore, if a rest is used in testing ammunition, the rest should touch the forearm of the rifle and if it is a narrow rest the rifle should always be rested at exacdy the same point. There arc some prone and bench rests, and very good ones, in which the barrel of the rifle is secured in a clamp; excellent shooting can be done with some of these. While the clamp on the barrel may effect the normal vibration of the barrel, the clamp is affixed at one point and the vibration, although abnormal is consistendy uniform.
Powder Gas Disturbances. But to get back to the movement of the bullet. After all parts of the bullet have attained an equal velocity, its movement along the bore of the vibrating barrel depends upon the kind and quantity of powder that is burned behind it. With normal charges of rifle powders, there is some acceleration of the bullet throughout the length of the barrel. This may be seen by cutting off segments of a rifle barrel and noting the drop in velocity of the bullet after each segment is cut off. But if a rifle is fired with a light charge of pistol powder, the powder will be consumed quickly and the bullet given
270 more or less of a bump or shove; as the bullet moves forward and the space behind it increases, the relatively small volume of gas available is unable to continue its accelerating effect on the bullet, the pressure drops rapidly, and the bullet may be retarded or slowed up by friction with the barrel before it leaves the muzzle. High pressures do not mean high muzzle velocities, as with a quick burning powder a gun can be burst before the bullet is out of the barrel, the gas pressure drops to nothing and the velocity of the bullet will be only that imparted to it before the burst occurred, less the retarding effect due to friction before it gets out of the muzzle of the gun.
As the bullet emerges from the muzzle of a barrel there is a tremendous gas disturbance behind it and the gasses, traveling at higher velocity than the bullet, envelop it. If the bases of all the bullets fired arc perfecdy flat, uniform and free from defects, the action of this expanding gas against the base of the bullet, when the latter is out of the gun, will be uniform from one shot to another. But if the bullet bases are appreciably defective, especially at the edges, the flight of such defective bullets will be affected by the gas.
For example, if a bullet has a nick or a serious casting defect in one side of its base, gas will escape through this as the bullet emerges from the muzzle of the gun, causing more or less tipping or "yaw" of the bullet. While such a bullet may by chance shoot into the same group with the rest, there is a much greater chance that it will not. In pistols and revolvers, where the pressures are low and the range at which such ammunition is fired is short, bullets with minor defects in their bases may appear to shoot fairly well for ordinary use, but not so with rifle bullets. The relatively high gas pressure behind rifle bullets, plus the longer ranges at which such bullets arc usually fired, will cause a considerable dispersion if bullets with defective bases are used.
Boat-tail bullets do not behave the same as flat base 271 bullets. A flat base bullet expands under the influence of the powder gasses, but the tendency of a boat-tail bullet is to collapse under the same circumstances. The tapered base of these bullets causes the gasses to act as a wedge and try to force their way in between the bullet and the barrel. For this reason, boat-tail bullets are made with very hard cores and the true base of a boat-tail bullet may be considered as the line of junction between the boat-tail taper and the cylindrical portion of the bullet. Special care must be taken in making such bullets that the beat-tail be concentric wich the point of the bullet and that this junction point be in a plane at right angles to the axis of the bullet.
The term "interior ballistics" applies to a sequence of events which terminate when the projectile has left the muzzle of the gun and the report has had time to reach *73 the car of and register itself on the sensibilities of the firer. The time required for the report to reach the ear is very short and is normally about equal to that required for the bullet to get beyond the effect of the expanding gasses.
A bullet suddenly projected in the atmosphere at a high velocity and with a severe gas disturbance behind it is subjected to rough treatment. In the early stages of its flight it may, and usually does, wobble considerably, at the same time deviating more or less from a plane through the axis of the bore. This instability of the bullet is sometimes referred to as "initial yaw." It is not the same in all bullets, as the shape of the bullet as wdl as the velocity at which it is travelling has a considerable effect on it. Any eccentric flight of the bullet may be further aggravated by a failure of the center of mass, or center of gravity, to coincide with the center of form.
When the bullet is passing through the rifling, which imparts rotation to it, it is forced to rotate around its center of form, being supported on all sides by the barrel, but when it emerges into the atmosphere, it will revolve around its center of mass. The effort to get these two 272 ccntcrs to coincide is one of the major problems of bullet manufacture. If a bullet jacket is thicker on one side than the other, or if the core is not of even density, the bullet will be eccentric in its flight throughout the legth of its trajectory. Likewise, a cast bullet can easily be slighdy heavier on one side than on the other if the alloy from which it is cast is not kept properly fluxed and stirred. As has been previously mentioned under the subject of bullets, cast bullets frequendy come from the mould slighdy out of round; these little irregularities are usually trued up by sizing the bullets, but even if they arc not and such bullets arc fired as cast, they will be sized up and trued to a greater or less extent when they arc forced through the bore of the rifle. On the other hand, a cast bullet or even a flat hase jacketed bullet is up against a tough proposition when fired in a rifle having a loose chamber, particularly one that is loose at the neck. for as has already been pointed out, there is a brief instant during which the neck of the cartridge is expanded, letting go of the bullet, but in which the bullet has not begun to move forward, after which the base of the bullet begins to move and finally the point, until the bullet slaps up into the throat of the rifle. During this brief instant when the bullet is beginning its forward movement, it is a matter of chance as to how it moves up into the throat of the barrel and its angular entrance, coupled with the expansion which takes place, can cause the most perfect bullet to become slighdy eccentric.
The disturbance of the bullet during its initial movement along its trajectory that is caused by its high velocity and the effect of the gas on it is temporar}', but any eccentricity of rotation caused by failure of the center of gravity and the center of form to coincide will be permanent throughout the flight of the bullet.
Bullets driven at high velocity will usually make slighdy oval holes in cardboard or paper screens placed at short distances from the muzzle of the gun which shows up graphically the effect of initial yaw, but this wobbling of the bullet is also influenced by the shape and sectional density o£ the bullet and the rate at which the bullet is rotating.
Speed of Rotation. The speed of rotation depends upon the pitch of the rifling and the muzzle velocity of the bullet. For example; if the rifling in a barrel has a pitch of one turn in ten inches and a bullet it fired from it with a muzzle velocity of 2,000 feet per second or 24,000 inches per second, the bullet will, in one second, make as many complete rotations as 10 inches will go into 24,000 inches, or 2,400 rotations per sccond. However, if the same bullet is fired from the same gun at a velocity of 3,000 f.s. its rate of rotation will be increased to 3,600 revolutions per second. Long bullets must be rotated faster than short bullets to keep them stable in flight or to give them what is known as gyroscopic stability, and a long bullet of small caliber will never become stable in flight if it is driven at too low a velocity.
Trajectory. A projectile emerging from the muzzle of the gun has a tough job ahead of it, forcing its way through the atmosphere. Living in it as we do, we are apt to think that the atmosphere is just about nothing at all, but in reality it is a dense, movable combination of gasses which have a serious retarding effect upon the flight of a bullet or any other object passing through it. The elastic quality of the atmosphere is responsible for a considerable part of the recoil of a firearm, the action of the gas emerging at high pressure from the muzzle of the gun compresses the atmosphere while the reaction is to force the gun to the rear and the effect on the bullet is to continually slow it up in its flight. As the bullet is unsupported and is acted upon by the force of gravity which is constant, its rate of fall towards the earth is approximately equal to the normal acceleration of gravity while its forward movement is continually decreased by the atmosphere. The trajectory is therefore, always a modified parabola.
Drift. This rotational friction causes the bullet to "drift'1 slighdy in the direction of its rotation or in the direction of the pitch of rifling. Because of the greater frictional area, a large caliber bullet will have a greater
The rotation of the bullet is not retarded to the same extent as its forward movement. The point of the bullet thrusts the atmosphere aside much as the prow of a boat thrusts water aside when passing through it, reducing the frictional effect of the atmosphere on the body of the bullet which after all does not involve any compression of the atmospheric medium. Bullets that have been fired at extreme ranges have been found to be routing after they had lost their forward motion and the tame is true of bullets that have been fired vertically, but the atmosphere does cause some retarding effect on the rotation.
amount of drift than a small caliber, all other things being equal. A bullet driven at high velocity from a rifle having a right hand twist may and frequendy does move to the left of a vertical plane through the axis of the bore, immediately after leaving the muzzle, but this is due to yaw and not to atmospheric friction. Once the yaw is overcome, the bullet will commence its drift in the direction of rotation.
The trajectory of a bullet with relation to the bullet's performance can be divided into three parts: the part where initial yaw occurs, the part where the bullet flies truly and the part where yaw sets in again. The part in which the initial yaw is pronounced, which varies with the design of the bullet and the velocity at which it is driven, but which in high velocity rifles usually extends several hundred yards from the muzzle of the gun. .As the remaining velocity of the bullet decreases, without appreciable decrease in the rotation, the bullet setdes down to a steady flight and is said to 4 go to sleep" and this even flight continues until the loss in velocity and atmospheric friction is so great that the bullet begins to lose its gyroscopic stability. When this point is reached the bullet begins to wobble, which increases its resistance to the atmosphere and it loses *75 velocity rapidly. As its rate of forward travel decreases, the effect of the action of gravity becomes more, pronounced and the bullet drops more rapidly towards the earth; if the angle of fire be high enough, the bullet will finally lose all forward velocity and drop straight down.
The calculation of trajectories and in fact, almost all exterior ballistic problems are dependant upon an accurate knowledge of the muzzle velocity of the bullet and a factor termed the "ballistic coefficient," which involves the sectional density of the bullet; that is, the bullet weight divided by the square of its diameter in inches plus a form factor having to do with the shape of the bullet This applies to flat base bullets. It is a difficult matter to accurately determine the ballistic coefficient of any bullet over its entire trajectory and this coefficient, when determined, can only be applied while the bullet is stable in flight, because the minute a bullet commences to wobble, or is unstable, or eccentric in flight, the air resistance is increased and the normal ballistic coefficient becomes useless.
The most important part of the trajectory and what might be termed the must useful pan is that in which the bullet is asleep and flying truly.
Except for the quantity of powder used in loading ammunition, which affects the muzzle velocity, the entire problem of handloading is one intimately related to internal ballistics. The first operation presenting itself is that of decapping, provided one is about to reload fired cases. The expelling of fired primers is of litde importance as far as methods arc concerned and any old way of getting the primer out is perfecdy satisfactory, provided the flash hole or vent in the cartridge case is not enlarged by the decapping pin or punch that is used. In extracting Berdan primers, care must be taken not to damage the anvil which is a part of the cartridge case, but we can practically skip that because Berdan primers arc not used in American ammunition nor are cartridge cases of the Berdan type reloaded to any great extent.
All reloading tools provide a means for expelling or pushing out fired primers in the form of a rod or punch that will pass freely through the mouth of the cartridge case. The end of the punch carries a small pin which in turn passes through the flash hole or vent in the case to force the fired primer out. These decapping punches are actuated by some mechanical means and sometimes operate under a very powerful leverage. This isn't a bad idea, because in military cartridges these primers are almost 277 always crimped in by having the brass in the head of the cartridge case compressed and upset around in the immediate vicinity of the primer pocket, making the mouth of the pocket smaller than the base. This makes primers especially difficult to push out. Sporting ammunition in other
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