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Rh 12-in. gun after some 150 or fewer rounds are fired with a full charge the barrel is worn so much as to need replacing. In the British service it is considered that the wear produced by firing sixteen half charges is equivalent to that of one full charge.

In small high-velocity guns the number of rounds with full charge which can be fired without replacing the barrel is considerably greater; while for low-velocity guns the number is higher still. In some guns this number appears abnormally high; in others of exactly similar type it may be low and for no apparent reason.

The first effect of the powder gases on the steel is a very characteristic hardening of the surface of the whole of the bore; so much is this the case that it is difficult to carry out any mechanical operation, except grinding, after a gun has been fired. When ignited the explosive contained in the chamber of the gun burns fiercely, and as the projectile travels along the bore the highly heated gases follow. The surface of the bore near the chamber is naturally the most highly heated and for the longest time; here too the rush of gas is greatest. There is in consequence a film of steel swept off from the surface, but this becomes less as the distance from the chamber becomes greater, owing to the abstraction of heat by the bore. It is a noticeable fact that only where a decided movement of gas takes place is there any erosion: thus, towards the breech end of the chamber where no rush of gas occurs there is no perceptible erosion, even after many rounds have been fired. Again, at the muzzle end there is very little erosion, as here the gases are in contact with the bore for a minute fraction of time.

As the firing proceeds, the interior surface of the bore, where the erosion is greatest, becomes covered with a network of very fine cracks running both longitudinally and circumferentially. The sides of these cracks in their turn become eroded and gradually fissures are formed. With the old black and brown powders these fissures were a feature of the erosion, while with the new type smokeless powders the eroded surface is usually smooth, and it is only after prolonged firing that fissures occur although fine cracks occur after a comparatively few rounds have been fired.

.—English: Nye, The Art of Gunnery (1670); Norton, The Gunner, showing the whole Practice of Artillerie (London, 1628); Sir Jonas Moore, Treatise of Artillery (London, 1683); Robins, New Principles of Gunnery (London, 1742); Hutton, Tracts (London, 1812); Sir Howard Douglas, R.A., Naval Gunnery (London, 1855); Mallet, Construction of Artillery (London, 1856); Boxer, Treatise on Artillery (London, 1856); Owen, Modern Artillery (London, 1871); Text-Book Rifled Ordnance (London, 1877); Treatise on Construction of Ordnance (London, 1879); Lloyd and Hadcock, Artillery: its Progress and Present Position (Portsmouth, 1893); Treatise on Service Ordnance (London, 1893–1904); Catalogue of Museum of Artillery in the Rotunda (Woolwich, 1906); Sir Andrew Noble, Artillery and Explosives (1906); Brassey, Naval Annual. United States: A. L. Holley, Ordnance and Armour (New York, 1865); E. Simpson, Ordnance and Naval Gunnery (New York, 1862); Resistance of Guns to Tangential Rupture (Washington, 1892); Annual Reports of Chief of Ordnance; Fullam and Hart, Text-Book of Ordnance and Gunnery (Annapolis, 1905); O. M. Lissak, Ordnance and Gunnery (New York, 1907). French: Jacob, Resistance et construction des bouches à feu (Paris, 1909); De Lagabbe, Matériel d’artillerie (Paris, 1903); Manuel du canonnier (1907); Alvin, Leçons sur l’artillerie (Paris, 1908). German and Austrian: Kaiser, Konstruktion der gezogenen Geschützröhre (Vienna, 1900); Indra, Die wahre Gestalt der Spannungskurve (Vienna, 1901). Italian: Tartaglia, La Nuova Scienta (Venice, 1562); Bianchi, Materiale d’artiglieria (Turin, 1905).

General Principles.—A field gun may be considered as a machine for delivering shrapnel bullets and high-explosive shell at a given distant point. The power of the machine is limited by its weight, and this is limited by the load which a team of six horses is able to pull at a trot on the road and across open country. For under these conditions it is found that six is the maximum number of horses which can work in one team without loss of efficiency. The most suitable load for a gun-team is variously estimated by different nations, according to the size of the horses available and to the nature of the country in the probable theatre of war. Thus in England the field artillery load is fixed at 43 cwt. behind the traces; France, 42·5 cwt., Germany 41·5 cwt., and Japan (1903) 30 cwt. This load consists of the gun with carriage and shield, the limber with ammunition and entrenching tools, and the gunners with their kits and accoutrements. The weights may be variously distributed, subject to the condition that for ease of draught the weight on the gun wheels must not greatly exceed that on the limber wheels. It is still usual to carry two gunners on seats on the gun axletree, and two on the limber. But a Q.F. gun capable of firing 20 rounds a minute requires to be constantly accompanied by an

ammunition wagon, and the modern tendency is to take advantage of this to carry some of the gunners on the wagon. Thus in the British field artillery two gunners are carried on the gun limber, two on the wagon limber, one on the wagon body and none on the gun. These five gunners, with the sergeant, called the No. 1, on his horse, make a full gun-detachment. Three wagons for each gun usually are provided, two of which, with the spare gunners and non-commissioned officers, are posted under cover at some distance behind the battery. Besides lightening the weight on the gun, the presence of the wagon allows the number of rounds in the limber to be reduced. The result of this redistribution of weights is that field artillery may now be equipped with a much heavier and more powerful gun than was formerly the case. A gun weighing 24 cwt. in action is about as heavy as a detachment of six can man-handle.

The power of a field gun is measured by its muzzle energy, which is proportional to the weight of the shell multiplied by the square of its velocity. The muzzle energy varies in different equipments from 230 to 380 foot-tons. Details of the power, weight and dimensions of the guns of the principal military nations are given in Table A.

A gun of given weight and power may fire a heavy shell with a low velocity, or a light shell with a high velocity. High velocity is the gunner’s ideal, for it implies a flat trajectory and a small angle of descent. The bullets when blown forward out of the shrapnel fly at first almost parallel to the surface of the ground, covering at medium ranges a depth of some 350 yards, as against half that distance for a low-velocity gun. Under modern tactical conditions a deep zone of shrapnel effect is most desirable. On the other hand, for a given power of gun, flatness of trajectory means a corresponding reduction in the weight of the shell; that is, in the number of shrapnel bullets discharged per minute. We have accordingly to compromise between high velocity and great shell power. Thus the British field gun fires an 18 ℔ shell with muzzle velocity of 1590 ft. per second, while the French gun, which is practically of the same power, fires a 16 ℔ shell with M.V. of 1740 f.s. Again, a shell of given weight may be fired either from a large-bore gun or from a small-bore gun; in the latter case the length of the shell will be proportionately increased. The small-bore gun is naturally the lighter of the two. But the longer the shell the thicker must its walls be, in order not to break up or collapse in the gun. The shorter the shell, the higher is the percentage of useful weight, consisting of powder and bullets, which it contains. We must, therefore, compromise between these antagonistic conditions, and select the calibre which gives the maximum useful weight of projectiles for a given weight of equipment. In practice it is found that a calibre of 3 in. is best suited to a shell weighing 15 ℔; and that, starting with this ratio, the calibre should vary as the cube root of the weight of the shell.

As to rifling, the relative advantages of uniform and increasing twist are disputed. The British field guns are rifled with uniform twist, but the balance of European opinion is in favour of a twist increasing from 1 turn in 50 calibres at the breech to 1 in 25 at the muzzle. Mathematically, the development of the groove is a parabola.

For field guns the favourite breech actions are the interrupted screw and the wedge. The latter is simpler, but affords a less powerful extractor for throwing out the empty cartridge case. This point is of importance, since cartridge cases hastily manufactured in war time might not all be true to gauge. Modern guns have percussion locks, in which a striker impinges upon a cap in the base of the metallic cartridge. All Q.F. guns have repeating trip-locks. In these, when the firing-lever or lanyard is pulled, the striker is first drawn back and then released, allowing it to fly forward against the cap. The gun is usually fired by the gun-layer; it is found that he lays more steadily if he knows that the gun cannot go off till he is ready. A field gun has to be sighted (see ) for laying (a) by direct vision (b) by clinometer and aiming-point (see ). The first purpose is served by the ordinary and telescopic sights; the second by the goniometric sight or the panorama sight. The