< Previous152P I S T O L S , R I F L E S A N D M A C H I N E G U N S British design, has a Continental calibre (9 mm.). On the other side there are the Colt revolvers with left-hand rifling, because Samuel Colt came to London to make his first revolvers and adopted the local custom. On returning to the U.S.A. he never changed back. It has been claimed that the reaction from a bullet accelerating in a left-hand direction turns a rifle to the right, that is away from the cheek of the firer. This is so, but the motion is insignificant because it is absorbed by the weight of the rifle. Apart from factory practice, there is no firm reason why the direction should be standardised, but it seems unneces sary to have the diversity we have. The only practical effect on the projectile is the direction of drift*, the magnitude of which is insignifi cant up to 1000 yards range, and only small at extreme machine gun ranges. A bullet spinning clockwise drifts to the right. Designers arrange to give a projectile the minimum rate of spin needed to launch it in stable flight, plus a small factor as an insurance. Since the rate of spin is retarded far less than the velocity along the trajectory, a projectile will be over-spun progressively, and this gives rise to yaw. A projectile is said to yaw when its axis make an angle with the tan gent to the trajectory instead of lying on it. Projectiles in stable flight often possess a small degree of yaw—it is unavoidable—but when it becomes excessive, air resistance to the passage of the projectile in creases, and the range, and possibly the accuracy, are impaired. A bullet may be yawed at shot ejection, and the amplitude increase immediately afterwards, but if the flight is stable the bullet will settle down as the yaw is damped out by frictional forces. An unstable projectile never settles, but becomes more and more unstable and finally flies wild. Fig. 69.—Diagram of a yawing bullet and the direction of drift. As the range increases the trajectory curves, and the axis of the bullet, now over-spun, makes an angle to the tangent of the trajectory at the point, (Figure 69). Gyroscopic action tends to keep the bullet axis * Drift is the lateral deviation of the trajectory due to the rotation of the projectile. Ya w . WWW.RIFLEMAN.ORG.UKRIFLES153 aligned as it was at the moment of departure, so the bullet point tends to rise above the trajectory. Air pressure on the underside of the bullet forces the point up, and as it is a gyroscope it turns at right angles to the applied force. Assuming that the bullet is spinning anti-clockwise, viewed from the rifle, then the point will move to the left. The air pressure now acts on the right side of the bullet tending to force the point to the left, but the gyroscopic action again causes it to move at right angles to the applied force, i.e. downwards. Precession about the bullet's centre of gravity continues, and the point describes a roughly circular path about the line of flight. Precession, in this instance, tends to keep the point of the projectile to the left of the trajectory, and it follows that air pressure on the right side of the bullet forces it bodily to the left, and consequently drift occurs. It is not possible in this small book to enter more fully into the complex motion, but it is hoped that this explanation will give the reader some idea of the facts. FURNITURE Furniture is the name given to the woodwork on a small arm, although the modern tendency is to employ other materials. The main flmction is to fill spaces and provide handholds. Italian Walnut is the best wood for rifles, but it needs careful seasoning and is expensive, so substitutes are continually being sought. The chief characteristics required of furniture are:— Immunity from oil and water, Plentiful supply available, Close grained and strong. Unaffected by climatic changes. No tendency to warp. Easy to machine. Comfortable to hold in all climates. Among the most satisfactory substitutes are:— Plastics, Laminated wood. Light alloy and steel pressings. Plastics, with fillers to increase the resistance to shock, were exten sively used by Germany in the last war, and have been tried by other nations. This material is heavier than wood and always feels more greasy. It is not so pleasant to handle, but has the advantage of rapid mass-production once the moulds have been made. Complex forms are simple to produce, and the moulds have a long life. L a m i n a t e d w o o d i s m a n u f a c t u r e d f r o m l a m i n a e b o n d e d w i t h a resinous glue. It may be similar to plywood, or the grain of the laminae may all run in one direction. Considerable strength, resistance to warping, oil, and water is achieved, but the product is not easy to machine. It is economic in manufacture because small pieces of wood can be used, owing to the multi layer construction (Example, E.G. 42 auto rifle). WWW.RIFLEMAN.ORG.UK1 5 4 . P I S T O L S , R I F L E S A N D M A C H I N E G U N S Pressed metals have been used extensively, particularly by Germany, and are satisfactory for mass production, but they are uncomfortable to handle in extreme climates. The weight factor is satisfactory, providing light alloys are used. Examples are the German M.P.40 machine carbine. Figure 58, and the Kar. 44 machine carbine with locked breech. Both these weapons are furnished in steel and are rather heavy. The accuracy of the No. 1 rifle depends to a large extent on the stocking up of the barrel in the fore-end, but later models of the S.M.L.E. have been redesigned to reduce this effect, and thereby facilitate assembly and maintenance. The packing of a rifle for competition shooting consists of filling the spaces between the barrel and fore-end in order to reduce barrel vibrations, and thereby improve the consistency of the shots. BARREL VIBRATIONS, JUMP, AND COMPENSATING RANGE A few notes on these theoretical aspects should give the reader an insight into the behaviour of rifles. The shock of the exploding charge in the breech causes a wave to travel along the barrel to the muzzle. The wave is propagated in steel at a much higher velocity than that of the bullet up the bore, and therefore reaches the muzzle first and influences the direction in which it is pointing at the moment of shot ejection. The important factor is the direction that the muzzle is moving in at the instant of shot ejection, and not whether it is plus or minus of the laid line. The only movements of importance are those in the vertical plane. The muzzle velocity of the .303 in. Mk. 7 bullet is approximately 2460 feet per second. A limit is set on the deviation of the individual shots from the mean velocity of a 10-round series at proof. This results in a maximum spread of about 120 feet per second between the fastest and slowest shots. A variation of velocity causes a change of range, but if the valuation of velocity is accompanied by a change in the angle of projection of opposite sign, some degree of compensation will occur. Take the case of a rifle muzzle moving up, due to barrel vibration, at shot ejection. The form of the wave motion does not vary with the change in velocity of the bullet, although the amplitude does to a small extent. A bullet moving below the mean velocity will take longer to traverse the bore, and will reach the muzzle later than a faster moving bullet. It will therefore be projected at a slightly greater angle of elevation, since the barrel is moving upwards. Conversely a fast bullet reaching the muzzle in a shorter time will emerge sooner, and at a lower angle of projection, thus the higher velocity is compensated for by the lower angle of projection, and vice versa. A rifle possessing these characteristics is said to be compensated. The No. 1 Rifle compensates at about 900 yards range, which means that a group of shots, aimed at the same point, will be smaller than would be the case, if there was no compensation. The ideal is to arrange for complete compensation a little short of the maximum effective range of a rifle, and so reduce the size of the group at the range where it would normally be spreading unduly. WWW.RIFLEMAN.ORG.UKRIFLES 155 If the barrel muzzle is moving down at the instant of shot ejection the errors will be additional, i.e., the slower bullet will be longer in the bore and will be projected at a lower angle of elevation than a faster bullet, therefore, it will range even shorter than the low velocity dictates. In the same manner the higher velocity bullet ranges further. A rifle with these characteristics is uncompensated, and would be expected to group badly at the longer ranges. The angle of jump is defined as the angle between the axis of the bore produced, as laid, and the line of departure of the projectile. It is evident that the barrel vibration causes jump, because the muzzle changes its angle during shot travel. The recoil of the weapon, although small during shot travel, may also influence the angle of jump. The attachment of a bayonet often alters the angle of jump and necessitates a change in the sight setting. All small arms are subject to jump. Jump is determined as follows. The weapon is fixed securely in a mounting and sighted through the bore on to a small aiming point on a screen at a range of 71.6 feet. At this range 4 minutes of angle subtends 1 inch. A group of shots is fired, and the mean point of impact carefully marked. The position where the shots would have struck, had there been no jump, is calculated from the formula, gravity drop = where g is the gravitational acceleration and t is the time of bullet flight over 71.6 feet, as measured by a chronograph. The formula gives the vertical drop due to gravity during the time of flight over 71.6 feet. A mark is made on the screen at the appropriate spot below the aiming point and the distance of the mean point of impact of the group is measured with respect to this mark. It is now a simple matter to calculate the angle of jump, using this distance. Vertical jump is counteracted when the sights are fitted at the factory. In any case jump is automatically compensated for when the sights of a weapon are zeroed, because the line of sight is matched to the group on the target (Chapter 8). WWW.RIFLEMAN.ORG.UKNext >