Coast-defense guns have followed the same lines of development as the naval. Mobile artillery and land guns for interior fortifica tions and siege operations have advanced on lines of their own. At the outbreak of war in 1914 the standard field piece in all armies was of about 3-inch calibre. The developments of recent years have been connected with the adaptation of heavy guns of all kinds to increase in size and use of howitzers and mortars, improvements in gunmounts, am munition and ammunition supply. The cation of rapid-fire mechanism to field guns was of little value until the recoil was con trolled so that the gun would not be thrown out of line by every shot. This was achieved by means of long recoil cylinders and a spade (a sharp projection for entering the ground) on the trail, while slight variations bf direc tion were corrected by fitting the gun-cradle on a pivot or slide to give a train of 6° to In the Boer War the 'British found much use for long-range guns of four to six inches in calibre, and in the Russo-Japan War the Japanese used 11-inch howitzers with great success in the investment and reduction of Port Arthur. These were heavy, coast-defense pieces and were moved with much difficulty from one concrete emplacement to another. After the conclusion of the war the Germans began experiments with large mortars of light construction designed for use in siege work and operations against fortifications of all kinds. In this they were followed by the Aus trians and French and in a small way by the other powers. On many of these the carriage and transporting trucks had a ring of broad flat pedals separately pivoted to the rim; on others the caterpillar belt was tried. High explosive shells were brought into general use about 1900 and great improvements were made in shrapnel, particularly as regards the ease of setting the fuse and certainty of its opera tion. Before passing to a more extended con sideration of existing land and naval ordnance, the question of erosion of the bore may erly be examined as it is common to guns of all kinds. It is due to several causes, but so far as large guns are concerned it depends chiefly upon (a) the temperature developed in the gun, (b) the chemical constitution of the powder gas, and (c) the character of the gun-tube. It is effected by (1) chemical action on the metal of the gun, (2) mechanical rub bing of the bore by the projectile, and (3) the cutting effect of gas moving with high velocity. When the charge explodes the temperature evolved is very high, possibly 2,500° C. for nitrocellulose and 3,000 C. for nitroglycerin, the actual maximum temperature of the gases depending upon the rapidity of combustion, the density of loading, loss of heat by conduction and radiation through the gun walls and the increase in volume (due to movement of the projectile) of chamber and bore before the instant of maximum pressure. The whole in terior of the bore behind the projectile is ex posed to this heat which tends to burn out the carbon from the steel and soften it. The sur face of the metal is oxidized, but if the film of oxide is not removed it protects the metal beneath. As the projectile moves forward its rotating band wipes the bore (especially the tops of the lands in the rifling) perfectly clean and so exposes it to renewed chemical action at each shot. In the course of time the combined
chemical and mechanical action washes away the lands at the rear end. Before the projectile starts—especially after the lands are much worn —a certain amount of gas escapes around it and causes irregular pitting, grooving and cutting, but this in large guns is almost imma terial compared to the washing away of the lands. Relative to the amount of carbon, hydrogen and nitrogen in its composition, nitroglycerin contains much more oxygen than nitrocellulose. This not only produces a higher temperature, but also furnishes a greater amount of oxygen to act on the metal of the gun. Powders containing nitroglycerin are therefore much more erosive than those made of pure nitrocellulose and the latter are be coming more and more common. Cup discs on the bases of projectiles have comparatively little effect on erosion, but they materially assist in preserving the muzzle velocity of pro jectiles fired from guns in which the lands are much worn.
The number of rounds which can be fired from a large gun before it needs relining varies within wide limits. It depends upon (a) On speed with which the rounds were fired, (b) pressure and temperature in the gun, (c) character of the metal of the bore, (d) chem ical composition of the powder, (e) means to keep up the velocity in worn guns, (1) opinion of the users of the gun as to when its power has been reduced beyond tolerance. All these factors are variable. In the case of 12-inch guns with velocities of 4800 feet per second; using projectiles of about 850 pounds and pure nitrocellulose powder; fired not more than six or eight times in one-half hour (the rounds may all be fired in three or four minutes if time for cooling occurs afterward) ; if cup gas-checks are used on the projectile; if slightly greater powder charges are used as the wear increases; and if the metal of the bore is of the best and most resistant type; then the number of rounds which they may be fired without the loss of more than two or three hundred feet of muzzle velocity is per haps 150 to 200. Fifty to 75 rounds- fired rapidly in battle might give the same destruc tive results. When the guns are condemned for inaccuracy the tube is removed and a new one inserted. This lining tube extends for about half the length of the bore from the powder chamber toward the muzzle and the cost of dismounting, relining, and remounting the gun is about 25 per cent of the cost of a new piece. To avoid delay reserve guns re place those which are removed from ships for relining. The life of larger or smaller calibres vary nearly in the proportion of the cube of the diameter of the bore. Thus, a 6-inch can fire about eight times as many shots as a 12 inch with the same loss of accuracy. But this rule is no more than a general guide as the length of time a high speed of fire is maintained is a most decisive factor in the result. Guns using powders containing nitroglycerin have a shorter life than those using nitrocellulose. If the nitroglycerin content is 25 per cent the number of rounds is at least 25 per cent less than is possible in a similar gun using a pure nitrocellulose powder. If the muzzle velocity of the projectile is cut from Z800 to 2,500 feet the per second, the length of life of the piece is extended 15 to 25 cent, depending upon the character of its use.