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by Ian Vernon Hogg
In the middle years of the 19th century, smoothbore field artillery was placed at a disadvantage by the adoption of rifled small arms, which meant that infantry weapons could now outrange artillery. It therefore became vital to develop rifling for artillery weapons as well. The advantages of rifling were well known, but the technical difficulties of adapting the principle to heavy weapons were considerable. Several systems had been tried; these generally involved lead-coated projectiles that could engage shallow rifling grooves or projectiles fitted with studs that would fit into deeper rifling. None had proved adequate.
In 1854 William Armstrong, an English hydraulic engineer, designed an entirely new type of gun. Instead of simply boring out a solid piece of metal, Armstrong forged his barrel from wrought iron (later from steel). He then forged a succession of tubes and, by heating and shrinking, assembled them over the basic barrel so as to strengthen it in the area where the greatest internal pressure occurred. The barrel was rifled with a number of narrow, spiral grooves, and the projectile was elongated and coated with lead. The gun was loaded from the rear, the breech being closed by a “vent-piece” of steel that was dropped into a vertical slot and secured there by a large-diameter screw. The screw was hollow so as to make it lighter and facilitate loading.
In 1859 the British adopted the Armstrong system for field and naval artillery. During this same period, the Prussians had been testing guns made by Alfred Krupp, and in 1856 they adopted their first Krupp breechloader. This was made of a solid steel forging, bored and then rifled with a few deep grooves, and its breech was closed by a transverse sliding steel wedge. The Krupp projectile had a number of soft metal studs set into its surface, positioned so as to align with the rifling grooves. In both the Armstrong and Krupp guns, obturation—that is, the sealing of the breech against the escape of gas—was performed by a soft metal ring let into the face of the vent piece or wedge. This pressed tightly against the chamber mouth to provide the required seal.
Meanwhile, the French adopted a muzzle-loading system designed by Treuille de Beaulieu, in which the gun had three deep spiral grooves and the projectile had soft metal studs. The gun was loaded from the muzzle by engaging the studs in the grooves before ramming the shell.
Armstrong guns were successful against Maoris in New Zealand and during the Opium Wars in China, but the development of ironclad ships in Europe demanded guns powerful enough to defeat armour, and the Armstrong gun’s breech closure was not strong enough to withstand large charges of powder. Therefore, in 1865 the British adopted a muzzle-loading system similar to that of de Beaulieu, since only this would provide the required power and avoid the complications of sealing the breech.
Through the 1870s guns, particularly coastal-defense and naval guns, became longer so as to extract the utmost power from large charges of gunpowder. This made muzzle loading more difficult and gave a greater incentive to the development of an efficient breech-loading system. Various mechanisms were tried, but the one that supplanted all others was the interrupted screw, devised in France. In this system the rear end of the bore was screw-threaded, and a similarly screwed plug was used to close the gun. In order to avoid having to turn the plug several times before closure was effected, the plug had segments of its thread removed, while the gun breech had matching segments cut away. In this way the screwed segments of the plug could be slipped past smooth segments of the breech, and the plug slid to its full depth. Then the plug could be revolved part of a turn, sufficient for the remaining threads to engage with those in the breech.
In the earliest applications of this system, obturation was provided by a thin metal cup on the face of the breechblock; this entered the gun chamber and was expanded tightly against the walls by the explosion of the charge. In practice, the cup tended to become damaged, leading to leakage of gas and erosion of the chamber.
Eventually a system devised by another French officer, Charles Ragon de Bange, became standard. Here the breechblock was in two pieces—a plug screwed with interrupted threads and having a central hole, and a “vent bolt” shaped like a mushroom. The stem of the bolt passed through the centre of the breechblock, and the “mushroom head” sat in front of the block. Between the mushroom head and the block was a pad of resilient material shaped to conform to the chamber mouth. On firing, the mushroom head was forced back, squeezing the pad outward so as to provide a gas-tight seal. This system, refined by a century of experience, became the principal method of obturation used with major-calibre artillery.
The alternative to this system was the sliding breechblock and metallic cartridge case pioneered by Krupp. Here the case expanded under the charge pressure and sealed against the chamber walls. As the pressure dropped, the case contracted slightly and could be withdrawn when the breechblock was opened. This system was embraced first by German gunmakers and later was widely used in all calibres up to 800 millimetres (about 31 inches). However, during World War II (1939–45), when the Germans were faced with metal shortages that threatened cartridge-case production, they developed a form of “ring obturation” so that bagged charges could be used. In this system an expandable metal ring was set into the face of the sliding breechblock, and its seating was vented in such a manner that some of the propellant gas was able to increase the pressure behind the ring and so force it into tighter contact. As improved in the postwar years, this system was adopted on a number of tank and artillery guns.
The lasting legacy of the Armstrong gun was the system of building up the gun from successive tubes, or “hoops”; this was retained in the rifled muzzle-loading system of the 1870s and was gradually adopted by other countries. Armstrong’s method not only economized on material, by distributing metal in accordance with the pressures to be resisted, but it also strengthened the gun.
An exception to the built-up system was practiced by Krupp. He bored guns from solid steel billets, making the barrels in one piece for all but the very largest calibres. In the mid-19th century it was difficult to produce a flawless billet of steel, and a flawed gun would burst explosively, endangering the gunners. A wrought-iron gun, on the other hand, tended to split progressively, giving the gunners warning of an impending failure. This was enough to warrant the use of wrought iron for many years, until steel production became more reliable.
The next major advance in gun construction came in the 1890s with wire-winding, in which one or more hoops were replaced by steel wire wound tightly around the tube. This gave good compressive strength but no longitudinal strength, and the guns frequently bent. Beginning in the 1920s, wire-winding was abandoned in favour of “auto-frettaging,” in which the gun tube was formed from a billet of steel and then subjected to intense internal pressure. This expanded the interior layers beyond their elastic limit, so that the outer layers of metal compressed the inner in a manner analogous to Armstrong’s hoops but in a homogenous piece of metal.
Until the 1860s guns were simply allowed to recoil along with their carriages until they stopped moving, and they were then manhandled back into firing position. The first attempt at controlling recoil came with the development of traversing carriages for coastal defenses and fortress guns. These consisted of a platform, pivoted at the front and sometimes carried on wheels at the rear, upon which a wooden gun carriage rested. The surface of the platform sloped upward to the rear, so that when the gun was fired and the carriage slid backward up the platform, the slope and friction absorbed the recoil. After reloading, the carriage was manhandled down the sliding platform, assisted by gravity, until the gun was once more in firing position, or “in battery.” To compensate for varying charges and, hence, varying recoil forces, the surface of the slide could be greased or sanded.
Control was improved by an American invention, the “compressor.” This consisted of loose plates, fitted at the sides of the carriage and overlapping the sides of the slide, which were tightened against the slide by means of screws. Another arrangement was the placing of a number of metal plates vertically between the sides of the slide and a similar set of plates hanging from the carriage, so that one set interleaved the other. By placing screw pressure on the slide plates, the carriage plates were squeezed between them and thus acted as a brake on the carriage movement.
American designers added to this by adopting a hydraulic buffer, consisting of a cylinder and piston attached to the rear of the slide. The fired gun recoiled until it struck the piston rod, driving the piston into the cylinder against a body of water to absorb the shock. British designers then adapted this by attaching the buffer to the slide and the piston rod to the carriage. As the gun recoiled, it drove the piston through water inside the cylinder; meanwhile, a hole in the piston head permitted the water to flow from one side of the piston to the other, giving controlled resistance to the movement. Return to battery was still performed by manpower and gravity.
The final improvement came with the development of mechanical methods of returning the gun to battery, generally by the use of a spring. When the gun recoiled, it was braked by a hydraulic cylinder and at the same time compressed a spring. As recoil stopped, the spring reasserted itself, and the gun was propelled back into battery. From there it was a short step to using compressed air or nitrogen instead of a spring, and such “hydropneumatic” recoil-control systems became standard after their introduction by the French in 1897.
Carriages and mountings
In 1850 carriages were broadly of two types. Field pieces were mounted on two-wheeled carriages with solid trails, while fortress artillery was mounted either on the “garrison standing carriage,” a boxlike structure on four small wheels, or on the platform-and-slide mounting previously described.
Coastal-defense artillery was the focus of most design attention in the 1870–95 period, since rapidly improving warships appeared to constitute the principal threat. The first major advance was a “disappearing carriage,” in which the gun was mounted at the end of two arms that were hinged to a rotating base. In the firing position, a counterweight or hydraulic press held the arms vertical, so that the gun pointed over the edge of the pit in which the mounting was built. On firing, recoil drove the gun back, causing the arms to pivot and sink the gun into the pit out of sight of the enemy, where it could be reloaded in safety. This type of mounting, in various forms, was widely adopted, but it was gradually realized to be excessively complicated in view of the practical difficulty of a ship’s gun being able to hit such a small target at long range. In most countries the disappearing mounting ceased to be built in the 1890s, though many of those already in position continued in use into the 1920s in Europe and into the 1940s in the United States.
In the 1890s the “barbette” mounting for coastal-defense guns became the preferred pattern. Here the mounting was in a shallow pit, protected from enemy fire, but the muzzle and upper shield were permanently in view, firing across a parapet that helped protect the gunners. This type of mounting was made practical by the development of hydraulic recoil control systems, which permitted the mounting to remain stationary while the gun, carried in a cradle, was allowed to recoil under control and then return to battery by spring or pneumatic power. The barbette remained the standard mounting for coastal-defense guns until their virtual disappearance after 1945.
Field carriage design entered a new era with the French 75-millimetre gun of 1897. This introduced an on-carriage hydropneumatic recoil-control system, a shield to protect the gunners, modern sighting, fixed ammunition, and a quick-acting breech mechanism—thus forming the prototype of what became known as the “quick-firing gun.” The idea was quickly taken up in other countries, and, by the outbreak of World War I (1914–18), such weapons were standard in all armies. Mountings for larger guns—up to about 155 millimetres, or 6 inches, in calibre—simply enlarged this basic design.
Up to World War I, with horses providing the standard motive power, it was necessary to design heavy field artillery so that gun and mounting could be dismantled into components, each of which would be within the hauling capacity of a horse team. The gun then traveled in its various pieces until it was reassembled at the firing point. Steam traction was attempted by the British during the South African War (1899–1902), but it was found that tractors could not take guns into firing position, as their smoke and steam was visible to the enemy. The gradual improvement of the internal combustion engine promised a replacement for the horse, but it saw relatively little application until the middle of World War I—and then only for heavier types of artillery.
The type of carriage developed for very heavy weapons was exemplified by that used for the German 420-millimetre howitzers—collectively known as “Big Bertha”—that were used to reduce the fortresses of Liège, Belg., in 1914. The equipment was split into four units—barrel, mounting with recoil system, carriage, and ground platform—which were carried on four wagons pulled by Daimler-Benz tractors. A fifth wagon carried a simple hoist, which, erected over the gun position, was used to lift the components from their wagons and fit them together. As the Great War continued, heavier howitzers and longer-ranging guns were made so large that they could not be split into convenient loads for road movement. Thus, the railway mounting became a major type for guns and howitzers up to 520-millimetre calibre. The heaviest guns could be assembled on large mountings, which in turn could be carried on a number of wheels so as to distribute the load evenly onto a railway track. The most impressive railway gun built during the war was the German 210-millimetre “Paris Gun,” which bombarded Paris from a range of 68 miles (109 kilometres) in 1918. Like many other railway guns, the Paris Gun was moved to its firing position by rail but, once in place, was lowered to a prepared ground platform.
Advances in carriage design after 1918 were relatively minor. The first was the general adoption of the split trail, in which two trail legs, opened to roughly 45°, were able to support a gun through a wider angle of traverse. Beginning in the 1960s came the gradual adoption of lightweight materials, culminating in the introduction by the British Vickers firm of a carriage built of titanium, which allowed a 155-millimetre howitzer to be helicopter-lifted. The 1960s also saw the introduction of auxiliary propulsion. Consisting of small motors that drove the wheels of towed guns, this permitted the gun to be moved from its firing position to a concealed or alternative position without calling up the towing vehicle. Propulsion motors also allowed the adoption of powered loading and ramming devices and powered assistance in opening trail legs and lowering platforms, thereby allowing the size of the crew to be reduced.
Azimuth and Range
In the 1850s the tactics of artillery were simple: the gun was positioned well to the front and fired across open sights straight at the enemy. The general adoption after the 1880s of long-range rifles firing smokeless-powder rounds rendered this tactic hazardous, and the South African War and Russo-Japanese War (1904–05) brought about a change in policy. Guns had to be concealed from the enemy’s view, and a system had to be found that allowed them to be aimed without a direct view of the target. The solution was the adoption of the “goniometric,” or “panoramic,” sight, which could be revolved in any direction and which was graduated in degrees relative to the axis of the gun bore. The gun’s position and that of the target were marked on a map, and the azimuth (the number of degrees clockwise from due north) between the two was measured. A prominent local feature, or a marker placed some distance from the gun, was then selected as an aiming point, and the azimuth between this and the gun’s position was also measured. Subtraction of one from the other produced the angle between a line to the aiming point and a line to the target. If this angle was then set on the goniometric sight and the gun shifted until the sight was laid on the aiming point, then the bore of the gun would be pointed at the target.
Once the azimuth was calculated, the range was arrived at by measuring off the map. This was then converted into an angle by consulting a table, calculated during development of the gun, on which ranges were tabulated against angles of elevation. The angle was then set on an adjustable spirit-level (a clinometer) attached to the elevating portion of the gun. Setting the elevation angle displaced a bubble from the level position, and elevating the gun until the bubble returned to the level position brought the gun bore to the correct elevation angle.
The combination of these two techniques was sufficient to fire a shell that would land close to the target. From there, a forward observer would instruct the gunner to change the azimuth and elevation until the shells struck the target. At this point the remaining guns of the battery, which would have followed the corrections and set them on their own sights, would join in to carry out the bombardment.
During World War I it became tactically desirable to bombard an enemy position without alerting him by ranging shots. This brought about the development of “predicted fire.”
While it is possible to determine azimuth and range from a map with accuracy, it is difficult to predict the actual performance of a fired shell. The density and temperature of the air through which the shell passes, the temperature of the propelling charge, any variation in weight of the shell from standard, any variation in the velocity of the shell owing to gradual wear on the gun—these and similar environmental changes can alter the performance of the shell from its theoretical values. Beginning in the 1914–18 period, these phenomena were studied and tables of correction were developed, together with a meteorological service that produced information upon which to base the corrections. This technique of predicted fire was slowly improved and was widely used during World War II, but the corrections were an approximation at best, owing to the simple tabular methods of applying the corrections. It was not until the introduction of computers in the 1960s that it became possible to apply corrections more accurately and more rapidly.
Until the second half of the 20th century, target acquisition—a vital part of fire control—was almost entirely visual, relying upon ground observers. This was augmented first by observation balloons and then, in World War II, by light aircraft, the object of both being to obtain better visual command over the battlefield.
In World War I two technical methods of targeting enemy gun positions were adopted—sound ranging and flash spotting. In sound ranging, a number of microphones were used to detect the sound waves of a gun being fired; by measuring the time interval between the passing of sound waves across different microphones, it was possible to determine a number of rays of direction that, when plotted on a map, would intersect at the position of the enemy’s gun. Flash spotting relied upon observers noting the azimuth of gun flashes and plotting these so as to obtain intersections. Both methods were highly effective, and sound ranging remained a major means of target acquisition for the rest of the century. Flash spotting fell into disuse after 1945, owing to the general adoption of flashless propellants, but in the late 1970s a new system of flash spotting became possible, using infrared sensors to detect the position of a fired gun.
Projectile, Powder, and Fuze
In 1850, round solid shot and black powder were standard ammunition for guns, while howitzers fired hollow powder-filled shells ignited by wooden fuzes filled with slow-burning powder. The introduction of rifled ordnance allowed the adoption of elongated projectiles, which, because of their streamlined forms, were much less affected by wind than round balls and, being decidedly heavier than balls of like diameter, ranged much farther. Yet the changing shape of projectiles did not at first affect their nature. For example, the shrapnel shell, as introduced in the 1790s by the Englishman Henry Shrapnel, was a spherical shell packed with a small charge of black powder and a number of musket balls. The powder, ignited by a simple fuze, opened the shell over concentrations of enemy troops, and the balls, with velocity imparted by the flying shell, had the effect of musket fire delivered at long range. When rifled artillery came into use, the original Shrapnel design was simply modified to suit the new elongated shells and remained the standard field-artillery projectile, since it was devastating against troops in the open.
Owing to the stabilizing spin imparted them by rifling grooves, elongated projectiles flew much straighter than balls, and they were virtually guaranteed to land point-first. Utilizing this principle, elongated powder-filled shells were fitted at the head with impact fuzes, which ignited the powder charge on striking the target. This in turn led to the adoption of powder-filled shells as antipersonnel projectiles. In naval gunnery, elongated armour-piercing projectiles initially were made of solid cast iron, with the heads chilled during the casting process to make them harder. Eventually, shells were made with a small charge of powder, which exploded by friction at the sudden deceleration of the shell upon impact. This was not an entirely satisfactory arrangement, since the shells generally exploded during their passage through the armour and not after they had penetrated to the vulnerable workings of the ship, but it was even less satisfactory to fit the shells with impact fuzes, which were simply crushed upon impact.
Between 1870 and 1890 much work was done on the development of propellants and explosives. Smokeless powders based on nitrocellulose (called ballistite in France and cordite in Britain) became the standard propellant, and compounds based on picric acid (under various names such as lyddite in Britain, melinite in France, and shimose in Japan) introduced modern high-explosive filling for shells. These more stable compounds demanded the development of fuzes adequate for armour-piercing shells, since friction was no longer a reliable method of igniting them. This was accomplished by fitting fuzes at the base of the shells, where impact against armour would not damage them but the shock of arrival would initiate them.
Time fuzes, designed to burst shrapnel shell over ground forces at a particular point in the shell’s trajectory, were gradually refined. These usually consisted of a fixed ring carrying a train of gunpowder, together with a similar but moveable ring. The moveable ring allowed the time of burning to be set by varying the point at which the fixed ring ignited the moveable train and the point at which the moveable train ignited the explosive.
During World War I these fuzes were fitted into antiaircraft shells, but it was discovered that they burned unpredictably at high altitudes. Powder-filled fuzes that worked under these conditions were eventually developed, but the Krupp firm set about developing clockwork fuzes that were not susceptible to atmospheric variations. These clockwork fuzes were also used for long-range shrapnel firing; inevitably, an undamaged specimen was recovered by the British, and the secret was out. By 1939 clockwork fuzes of various patterns, some using spring drive and some centrifugal drive, were in general use.
World War I also saw the development of specialized projectiles to meet various tactical demands. Smoke shells, filled with white phosphorus, were adopted for screening the activities of troops; illuminating shells, containing magnesium flares suspended by parachutes, illuminated the battlefield at night; gas shells, filled with various chemicals such as chlorine or mustard gas, were used against troops; incendiary shells were developed for setting fire to hydrogen-filled zeppelins. High explosives were improved, with TNT (trinitrotoluene) and amatol (a mixture of TNT and ammonium nitrate) becoming standard shell fillings.
World War II saw the general improvement of these shell types, though the same basic features were used and flashless propellants, using nitroguanidine and other organic compounds, gradually took over from the earlier simple nitrocellulose types. The proximity fuze was developed by joint British–American research and was adopted first for air defense and later for ground bombardment. Inside the proximity fuze was a small radio transmitter that sent out a continuous signal; when the signal struck a solid object, it was reflected and detected by the fuze, and the interaction between transmitted and received signals was used to trigger the detonation of the shell. This type of fuze increased the chances of inflicting damage on aircraft targets, and it also allowed field artillery to burst shells in the air at a lethal distance above ground targets without having to establish the exact range for the fuze setting.
After 1945 the proximity fuze was improved by the transistor and the integrated circuit. These allowed fuzes to be considerably reduced in size, and they also allowed the cost to be reduced, making it economically possible to have a combination proximity/impact fuze that would cater to almost all artillery requirements. Modern electronics also made possible the development of electronic time fuzes, which, replacing the mechanical clockwork type, could be more easily set and were much more accurate.
Nuclear Shells, Guided Projectiles, and Rocket Assistance
Nuclear explosive was adapted to artillery by the United States’ “Atomic Annie,” a 280-millimetre gun introduced in 1953. This fired a 15-kiloton atomic projectile to a range of 17 miles, but, weighing 85 tons, it proved too cumbersome for use in the field and was soon obsolete. In its place, nuclear projectiles with yields ranging from 0.1 to 12 kilotons were developed for conventional 203-millimetre howitzers. Soviet major-calibre artillery was also provided with nuclear ammunition.
The 1970s saw the first moves toward “improved conventional munitions.” These were artillery projectiles carrying a number of subprojectiles—antipersonnel bombs or mines or antitank mines—that could be fired from a gun and would be opened, by a time fuze, over the target area to distribute the submunitions. This increased the destructive power of an artillery shell by a large amount and allowed field artillery to place obstacles in the path of enemy tanks at a range of several miles. A further step was the development of guided projectiles. With the 155-millimetre Copperhead, a U.S. system, a forward observer could “illuminate” a target with laser light, a portion of which would be reflected and picked up by sensors in the approaching shell. The greater part of the shell’s flight would be entirely ballistic, but in the last few hundred yards it would be controlled by fins or other means, which, guided by the laser detection system, would “home” the shell onto the target.
In order to improve the range of guns, rocket-assisted projectiles were developed, with moderate success, by the Germans during World War II, and they were the subject of further development in succeeding years. Rocket assistance had certain drawbacks—notably, the loss of payload space in the shell to the rocket motor. A system designed to solve this problem was “base bleed,” in which a small compartment in the base of the shell was filled with a piece of smokeless propellant. This would burn during flight, and the emergent gases would fill the vacuum left behind the shell in its passage through the air, reducing aerodynamic drag on the shell and improving the range by about 25 to 30 percent.
The mortar declined in importance during the 19th century but was restored by World War I, when short-range, high-trajectory weapons were developed to drop bombs into enemy trenches. Early designs in that conflict ranged from the 170-millimetre German Minenwerfer (“mine thrower”), which was almost a scaled-down howitzer, to primitive muzzle-loading devices manufactured from rejected artillery shells. The prototype of the modern mortar was a three-inch weapon developed by the Englishman Wilfred Stokes in 1915. This consisted of a smooth-bored tube, resting upon a baseplate and supported by a bipod, that had a fixed firing pin at its breech end. The bomb was a simple cylinder packed with explosive and fitted with a shotgun cartridge at the rear; its fuze was adapted from a hand grenade. When the bomb was dropped down the barrel of the mortar, it fired automatically as the shotgun cartridge struck the fixed firing pin. The bomb was unstable in flight but sufficiently accurate for its purpose, and it was soon replaced by a teardrop-shaped bomb with fins at the rear, which lent greater stability and accuracy. The Stokes mortar was rapidly adopted or copied by all belligerents.
Some later mortars were built with rifled barrels, since these provided better sealing of the propellant gas and greater stability and accuracy owing to the spin imparted to the bomb. The difficulty here was to arrange for the bomb to be drop-loaded freely and yet engage the rifling once the propelling charge exploded. The U.S.-made M30, a 107-millimetre rifled mortar, used a saucer-shaped copper disk behind the bomb that flattened out into the rifling under gas pressure and provided obturation. In the 120-millimetre French Hotchkiss-Brandt type, a prerifled copper driving band, wrapped around the bomb, expanded under gas pressure and engaged the grooves in the barrel.
Heavy weapons and the problem of fire control
The development of antiaircraft guns began in 1909. The manufacture of suitable guns and mountings was not difficult at that time, but the fire-control problem, involving a target moving in three planes at high speed, was almost insoluble. The first fire-control system used complex gun sights that aimed the gun well in front of the target in order to give the shell time to reach it. The first projectiles were shrapnel, since scattered lead balls were sufficient to damage the aircraft of the day.
During World War I, attacks by German zeppelins led the British to produce incendiary shells. Forced to correct fire by visual methods, they fitted the shells with tracer devices, which, by leaving a trail of flame and smoke, indicated the shell’s trajectory in the air. The French invented the “central post” system of fire control, in which an observing instrument in the centre of the battery calculated the aiming information, which was then passed on to the guns. This removed complex sights from the weapons and reduced the number of skilled operators required in a battery. Early warning of approaching aircraft was by visual means and acoustic devices.
In the 1920s work began on the design of “predictors,” mechanical computers that could be given the course, height, and speed of the aircraft as well as the ballistic constants of the gun and could then calculate the gun data necessary to place the shell in the future position of the aircraft. These represented a significant advance in antiaircraft fire, but they still relied upon raw data provided by visual acquisition and tracking. In World War II, radar brought more accurate and timely acquisition and tracking, and the gradual adoption of electrical, rather than mechanical, predictors produced more accurate fire control. Also, rapid-loading and fuze-setting devices were incorporated into gun mountings so that a high rate of fire could be achieved.
The proximity fuze removed the need for fuze setting and thus speeded up the rate of fire, until it was possible for guns of 90- to 100-millimetre calibre to fire at rates up to 60 rounds per minute. However, in the 1950s, when all these techniques were perfected, guided surface-to-air missiles became practical, and, in all major countries except for the Soviet Union, the use of medium and heavy air-defense guns ceased.
Light air-defense guns, of calibres from 20 to 40 millimetres, were developed in the 1930s for protection against dive bombers and low-level attack. The most famous of these was a 40-millimetre gun sold by the Swedish firm of Bofors. Virtually an enlarged machine gun, this fired small exploding shells at a rate of about 120 rounds per minute—fast enough to provide a dense screen of fragments through which the aircraft would have to fly. Fire control was largely visual, though some guns were equipped with predictors and power control.
The advent of lightweight missiles also threatened to render the light gun obsolete in the 1950s, but two decades later the development of electro-optical sights, using television and thermal-imaging technology and allied to computers and powered mountings, led to a resurgence of this class of weapon. In Egyptian hands in October 1973, the Soviet ZSU-23-4, consisting of four 23-millimetre guns mounted on a tracked vehicle, shot down many Israeli fighters over the Sinai Peninsula. The Bofors firm mounted its guns on wheeled vehicles, and the United States fielded a mobile system called Vulcan, which consisted of a six-barreled, Gatling-type gun firing 20-millimetre ammunition.
The development of dedicated weapons for attacking tanks began in earnest in the 1930s. These were all in the 20- to 40-millimetre class, were mounted on light, two-wheeled, split-trail carriages, and were adequate against the tanks of the day. As tanks acquired heavier armour during World War II, so the guns became larger, eventually reaching 128 millimetres in calibre. The guns themselves did not generally demand new technology, but the development of ammunition had to break new ground.
The initial antitank projectile was a solid shot of hardened steel, and, in order to penetrate thicker tank armour, it was fired at higher and higher velocities. However, at a striking velocity of about 2,600 feet (800 metres) per second, steel shot shatters upon impact instead of penetrating. In order to overcome this, projectiles of tungsten carbide were used. The Germans designed a gun with a bore actually tapering in diameter from breech to muzzle, and for ammunition they constructed a projectile with a tungsten core and a soft metal body that would deform and squeeze in the reducing bore. The combination of reduced base area and constant gas pressure increased the projectile’s velocity, and the “taper-bore” or “squeeze-bore” gun proved formidable. Guns with tapering calibres of 28/20, 41/29, and 75/55 millimetres were developed, but wartime shortages of tungsten led to their abandonment after 1942. In 1944 Britain perfected “discarding-sabot” projectiles, in which a tungsten core was supported in a conventional gun by a light metal sabot that split and fell free after leaving the muzzle, allowing the core to fly on at extremely high velocity.
An alternative method was to use high explosives in the form of shaped-charge or squash-head projectiles. The shaped charge was an explosive formed into a hollow cone and lined with heavy metal; upon detonation, the explosive gases and molten metal formed a high-velocity jet capable of punching through armour. The squash-head shell used a plastic explosive filling, which, deposited on the armour and then detonated, drove a shock wave through the plate. This resulted in the failure of the inner face and the ejection of a massive slab of metal into the tank.
Heavy antitank guns relying upon high-velocity projectiles largely fell into disuse after 1945, but the technology was perpetuated in the main armament mounted on tanks (see tank). Explosive-energy projectiles were also used on tanks as well as on recoilless guns.
Military inventors were long attracted by the prospect of abolishing recoil, since achieving this meant doing away with the gun’s heavy recoil system and lightening the carriage. The first to succeed was Commander Cleland Davis of the U.S. Navy, who in 1912 developed a gun with a single chamber and two opposite barrels. One barrel carried the projectile, the other an equal weight of grease and lead shot. The explosion of the central cartridge ejected both loads, and, since the recoils had the same weight and velocity, they canceled each other out and the gun remained stationary. Davis’ idea was adopted in 1915 by the Royal Naval Air Service, which ordered guns of 40, 57, and 75 millimetres for arming aircraft against airships and submarines. Few were made, however, and there appears to be no record of their use in combat.
If the Davis principle were taken to its logical ends, the countershot could be half the weight and twice the velocity of the principal projectile or any other combination giving the same momentum; at its ultimate, the countershot could simply be a cloud of high-velocity gas. This was the system upon which recoilless guns of up to 105 millimetres were developed during World War II. The cartridge cases of these weapons had a weakened section that ruptured on firing, allowing about four-fifths of the propellant gas to be exhausted to the rear of the gun. There they passed through a venturi, a nozzle with a constricted portion that increased the gas velocity and so balanced the recoil generated by the projectile. The back-blast caused by the exhausted gases made these weapons difficult to emplace and conceal, but after 1945 they were universally adopted as light antitank weapons.