In World War II, as in every major war of the past several centuries, naval guns played an outstanding part toward the achievement of victory. Even with the advent of such super-weapons as guided missiles and the atomic bomb, it is likely that the naval gun will remain a vital weapon in our national defense, and some observations on the development of high- performance naval guns may be in order here.
Guns in naval service are used principally for one or more of the following purposes: (a) surface fire against ships, (b) shore bombardment, or (c) anti-aircraft fire. Versatility of usage is desirable, and is sometimes the predominant requirement, but no single design of gun is likely to be optimum for all three purposes. Our 5-inch 38-caliber gun proved itself highly effective in all three applications, but for any single type of target there could undoubtedly have been developed an alternative, more effective gun. Effectiveness against armored ships calls for the use of heavy, thick-walled projectiles with fairly high striking velocity, in order to obtain penetration. For shore bombardment, large thin-walled projectiles carrying large amounts of explosive are desirable, and often low velocity is preferred in order to provide the steep angle of fall resulting from a high trajectory. The prime requisites of effective anti-aircraft fire include an extremely high firing rate and short time of flight to the target, because of the difficulty of hitting high-speed aerial targets.
In World War I, surface fire was strongly predominant in naval warfare, since aircraft had not yet become a serious menace to naval vessels, and shore bombardment was seldom found necessary (with the notable exception of the seaborne attack on shore positions at the Dardanelles). In World War II, the requirements of naval guns were greatly broadened by the advent of high-speed aircraft carrying bombs and torpedoes, and by the necessity of attacking enemy-held shores. Existing guns were found readily adaptable for shore bombardment, by the provision of special ammunition, but experience showed that the minimizing of damage from determined air attacks would require guns and ammunition of maximum performance, especially designed for anti-aircraft use.
Requirements of an Effective Gun
For anti-aircraft fire or for any other purpose, the effectiveness of a gun depends principally upon its ability to hit the target, and upon the destructiveness of its projectiles. Ability to hit the target in turn is affected by three main factors: (a) the accuracy with which the future position of the target can be predicted, (b) the accuracy with which the gun can cause its projectiles to follow their expected trajectories, and (c) the rate of fire. The accurate prediction of target position is a fire control problem which will not be considered here, but it is evident that minimizing the time of flight of the projectile to the target will simplify such prediction. For firing against land targets, accurate prediction is simple, and for firing against ships it is not too difficult, but for firing against high-speed maneuvering aircraft it becomes a monumental task. To assist in the antiaircraft fire-control problem, then it is essential that the gun designer provide for sending the projectiles to the target in the least possible time of flight (see Fig. 1), consistent with related considerations. For any- given type of projectile, short time of flight corresponds with high “muzzle velocity” as the projectile leaves the gun, so in the improvement of anti-aircraft guns there is a trend toward higher and higher muzzle velocities.
Attainment of High Muzzle Velocity
Projectiles may be given high velocities by several means, some of which require only guns of conventional design, and some of which involve radical innovations in both the gun and ammunition (see Fig. 2). All of the following high-velocity systems were used either in action or experimentally in World War II:
(a) Conventional guns firing light-weight full-diameter projectiles. It was found that muzzle velocities well over 4000 fps (feet per second) could be produced by this method, where the muzzle velocity for the usual projectiles in the same gun was about 2700 fps. At short range, this gave shorter time of flight to the target and increased the armor- penetrating ability, which resulted in the application of this system for army anti-tank service. However, for the longer ranges usually prevailing in naval usage, against aircraft or other targets, the light-weight, full-diameter projectiles defeat their own purpose, because they are quickly slowed down by air resistance. As a result, they then have longer time of flight and less impact velocity than heavier projectiles of the same diameter fired at lower muzzle velocity.
(b) Conventional guns firing light-weight sub-caliber projectiles. To reduce the slowing- down effect of air resistance, a light-weight projectile can be made with its diameter much smaller than the bore of the gun, in which event the projectile is said to be “subcaliber.” This sort of projectile, to be fired from a conventional gun, is fitted with a light-weight bushing (usually called a “sabot”) which fits the gun bore, and which is automatically discarded when the projectile leaves the muzzle of the gun. The German army fired “sabot” projectiles at higher than 5000 fps muzzle velocity from guns as large as 11-inch, and also fired high-velocity sabot projectiles from 90-millimeter anti-aircraft guns. For naval usage, the sabot projectile has a distinct disadvantage in that the discarded sabot is itself a dangerous missile which could damage nearby friendly ships. However, sabot projectiles can provide short time of flight and good accuracy, which at times might compensate for the danger from discarded sabots.
(c) High-strength or extra-long guns firing conventional projectiles. By increasing the powder pressure and/or by increasing the length of the gun barrel, high velocity can be imparted to projectiles of conventional design. The former requires a stronger gun barrel than usual, the latter requires more shipboard space, and either one requires the gun to be heavier than for firing at ordinary velocities. The attainable velocity is limited by the pressure which the gun and projectiles can withstand, and by the pressure which can be developed with available types of powder.
(d) Tapered-bore guns firing “skirted” projectiles. The advantages of the sabot projectile, without its attendant danger to nearby friendly forces, have been obtained by using a gun whose bore tapers down toward the muzzle. It fires a projectile whose body is small enough to pass through the muzzle of the gun, and which is fitted with collapsible “skirts” that provide a gas-tight seal as the projectile travels along the tapered bore. High velocity is readily obtained, because the base of the projectile subjected to powder pressure is large until the projectile gets near the muzzle of the gun. Then the tapered bore squeezes the diameter of the projectile down to such a size that the air resistance during flight will be small. Tapered-bore anti-tank guns used by the Germans had muzzle velocities as high as 4700 fps, and the emergent diameters of projectiles varied from 20 millimeters up to more than 75 millimeters in different guns. Tapered-bore guns for antiaircraft use were also developed during the war, but apparently did not get into action.
(e) Guns firing rocket-assisted projectiles. Some large German army guns fired projectiles in which a rocket action was initiated in the projectile during its flight. By this means, the maximum range of the gun was considerably increased and the time of flight to any given range reduced, but the necessity of carrying rocket propellant reduced the “pay load” of high explosive which the projectile could carry. It appears doubtful if rocket- assisted projectiles could consistently provide the accuracy obtainable from other projectiles, but they are worthy of consideration for firing at very long ranges.
The foregoing briefly describes five methods by which it has been demonstrated that guns can impart very high velocities to projectiles. The reference only to German usage of such weapons does not mean that similar or more advanced developments were not made by other nations, but the latter cannot be discussed here because of security restrictions.
Disadvantages of High-Velocity Guns
Because of the better chance of hitting a moving target with a projectile having a short time of flight, it might appear self- evident that for maximum effectiveness a gun should have the highest possible muzzle velocity. However, for shipboard use in particular, this is not necessarily true. Considering any particular weight of projectile, a gun which can fire it at, say, 4000 fps must be larger and heavier than an equally well designed gun which can fire it only at an ordinary velocity such as 2700 fps. Since the number of guns which a ship can carry is limited by the size and weight of the guns, the increased effectiveness of a high-velocity gun must be very considerable before it compensates for the increased size and weight required. Because of the additional powder required, the cartridges must be larger than for conventional guns, thereby complicating the ammunition handling problem and making it more difficult to design the gun for a high rate of fire. Further, high-velocity guns wear out much quicker than ordinary guns, if made of the same materials. Therefore, it is only through very careful study of the relative weight, size, life, and effectiveness of high-velocity and ordinary-velocity guns, that the optimum muzzle velocity for naval guns can be ascertained.
Light-Weight “Recoilless” Guns
It has been found practical to eliminate the recoil of a gun by allowing some of the burning powder gases to blow backward through a nozzle in the breech of the gun. This action permits the gun mount to be very light in weight, and has been found advantageous in land service. The Germans used such guns, and the successful development of recoilless guns has been announced by the U. S. Army. The principal disadvantage of these guns is the tremendous rearward blast of flame, which is often acceptable for land service, but which is difficult to cope with aboard ship, if the gun is to have a wide arc of fire or to fire at high elevations.
Design Aspects of High-Performance Gun Barrels
Regardless of the type of gun and projectile to be used, the attainment of maximum performance from minimum size and weight requires accurate knowledge of strength and stress conditions in the gun barrel. The gun design methods prevailing before World War II were adequate for the conventional guns then being constructed, but marked advances in design methods were essential in the development of guns of greatly increased performance. It had been customary to design gun barrels of sufficient strength to withstand the powder pressure, with a reasonable margin of safety, without taking into account any stresses that the projectile itself might produce in the gun. Any such stresses, it was assumed, would be taken care of by the margin of safety which was allowed. However, in minimizing the size and weight of high- performance guns, it was necessary to reduce the margin of safety to a minimum. During experiments toward this end, the startling discovery was made that the copper band on the projectile, fitting tightly in the rifled bore of the gun, could and often did produce higher stress in the gun than did the powder pressure (see Fig. 3). This discovery almost revolutionized gun barrel design methods. A gun barrel firing a projectile must evidently be likened to an ostrich swallowing an orange! Advanced methods of stress analysis had to be developed, so that lightweight, high-strength gun barrels could be reliably designed to withstand not only the powder pressure but also the “ostrich-and- orange” effect of the projectile traveling down the bore. The problem was undertaken and solved, so that finally every last ounce of performance could be safely squeezed from a gun barrel.
Gun Life
In evaluating the effectiveness of a naval gun, there must be considered not only its performance when new, but also its ability to maintain good performance over an adequate length of life. In machine guns, the end of satisfactory performance is often directly observable from the erratic flight of tracer projectiles, and in extreme cases with small machine guns it has even happened that a projectile would emerge from the side of the barrel before reaching the muzzle. However, particularly in larger guns, it is highly desirable that the barrels be replaced before the effectiveness is seriously impaired. There are three principal criteria of the end of satisfactory performance of gun barrels: accuracy loss, velocity loss, and fuze action. Reduced accuracy cuts down the frequency of hitting the target; velocity loss diminishes both the maximum range and the armor penetration; and the premature detonation or nonfunctioning of projectile fuzes destroys effectiveness and may endanger friendly personnel. After extensive study and tests of many guns, reliable correlation has been established between the measurable wear in gun bores and the performance of the guns, so that the guns may be replaced under convenient conditions shortly before the quality of their performance is endangered.
Safety Considerations
The prolonged rapid fire of naval guns in World War II brought to light the necessity for special safety precautions not previously observed. It had long been realized that a misfire in a hot machine gun can result in the misfired cartridge being fired possibly a minute later by the heat absorbed from the gun barrel. Such occurrences had not been observed in larger guns, but World War II experience showed that even conventional guns as large as 5-inch were not immune to this action, called “cooking off.” High- performance guns, with their higher operating temperatures, would be even more susceptible. A cook-off of a cartridge containing a non-explosive projectile (as in small machine guns) cannot cause the barrel to burst, but endangers anyone who might be attempting to extract the misfired cartridge to return the gun to action. If the projectile itself is explosive, as is ordinarily the case in larger guns, the danger to personnel and materiel is much greater. It may be the explosive in the projectile which cooks off first, before the powder in the cartridge case, in which event the entire gun may be violently destroyed. The avoidance of disastrous cookoffs has been worked on from two aspects: first, the prevention of misfires, and second, the development of the safest procedures for clearing misfired ammunition from hot guns. Toward the prevention of misfires, the design of ammunition primers and of firing mechanisms has been made as fool-proof as possible. For clearing live ammunition from a hot gun, the safest method is to fire it out promptly by any auxiliary means at hand. When prompt firing is impossible, the procedure to be prescribed must take into account the relative seriousness of allowing the cook-off to take place in the gun, or of accepting whatever dangers attend extraction of the misfired ammunition. In combat, it is usually paramount that the gun be returned to action at the earliest possible moment, while in target practice the safety of personnel is the main consideration. So many factors are involved that no simple doctrine can satisfactorily cover all situations.
Summary
The foregoing discussion has outlined some of the highlights of special considerations involved in the design and usage of high-performance naval guns. Such guns may be of essentially conventional design, or they may involve novel features such as tapered bores, sabot projectiles, or rocket-assisted projectiles. In any event, the attainment of maximum performance from minimum size and weight requires special methods of designing for strength, and such methods have recently been developed. The severe wear resulting from high-velocity operation requires that particular attention be paid to obtaining adequate gun life. And since high-performance guns develop unusually high temperatures in the bore, much attention must be and has been given to the establishment of adequate safety precautions relevant to the disposal of live ammunition left in hot guns from misfires or other causes.