Chapter 6 of NavPers 10797-A, Naval Ordnance and Gunnery, Volume 1 explains how a modern naval gun barrel is built to withstand the enormous pressures of firing — from the fundamental stresses described by Lamé's law, through the built-up gun and autofrettage construction methods, to the science of interior ballistics governing what happens inside the gun from ignition to muzzle exit. The chapter also covers erosion: its causes, measurement with the star gage, control by chromium plating and cooler propellants, and prediction of velocity loss. It concludes with World War II-era experiments in attaining high muzzle velocity through subcaliber, tapered-bore, and rocket-assisted projectiles.
A. Introduction
6A1. Scope of this chapter
The preceding chapter has already defined a gun as a tube designed to discharge a projectile at high velocity by the gas pressure produced by a propellant in the tube. Commonly, the term gun applies to the entire assembly of which the barrel is but one part.
In this chapter, gun, tube, or barrel designates the gun tube only, and not the remainder of the gun assembly, which includes, in addition, the mount and other parts described in the preceding chapter.
This chapter is concerned with gun barrel construction and maintenance, and with interior ballistics — what happens inside the gun when it is fired.
B. Elements of Gun Design and Maintenance
6B1. Modern requirements for gun power
Present requirements for guns demand muzzle velocities of from 2,500 to 3,500 fps. Lower velocities give less striking energy. More important still, a projectile fired at low velocity would describe a curve so high in the air, for long ranges, that hits could not be made unless the range were known with great accuracy. Since the accurate determination of range is a critical problem in naval gunnery, the high-power gun is a necessity. High velocity of a projectile is produced, of course, by high pressure upon it while traveling through the bore.
A gun may be considered as a tube designed to withstand a given pressure from within. In constructing such a tube, we must first consider what pressures it will have to withstand at the various points of its length, and then make it strong enough to insure perfect safety. The bore should also be of such material as to stand the wear and tear of firing a large number of rounds without being so damaged by expansion or abrasion as to interfere with the shooting.
6B2. Stresses in a gun cylinder
Considering a gun only as a cylinder, we find that the two principal stresses (fig. 6B1) to which such a cylinder is subjected upon the explosion of a charge are:
- A circumferential or tangential stress or tension, coupled with a radial stress, tending to split the gun open longitudinally.
- A longitudinal stress tending to pull the gun apart in the direction of its length.
Experiments have shown that the greatest stress on the metal of the gun is the tensile stress set up in the direction of its circumference by powder gas pressure. In addition, the gun also experiences a longitudinal stress of relatively small value. If this longitudinal stress may be considered constant (and in guns it may be so considered without great error) we may lay down the first of “Lamé's laws,” as follows:
At any point whatever, in a cylinder under fluid pressure, the sum of the tangential tension and the radial pressure varies inversely as the square of the radius.
This law says, in effect, that in a simple hollow cylinder under internal pressure, points in the metal close to the bore experience a large proportion of the stress, whereas those at a greater radius experience only a small proportion. This means that in a simple hollow cylinder composed throughout of metal of homogeneous physical properties, we soon reach a limit beyond which any thickness of wall aids but little in enabling the cylinder to withstand pressure.
Hence a modern gun would not be sufficiently strong to withstand the required pressure if made of a single simple hollow cylinder, however thick. But the gun must be built on a principle which will enable it to withstand more internal pressure than could be withstood by the simple cylinder type of construction. The problem is to make the outer layers take a proper proportion of the stress. In one modern solution to the problem, the gun is constructed of layers of metal. The layers nearer the bore are held under an initial compression by the tension of the outer layers. Thus, when the gun is fired, the inner layers must first be expanded sufficiently to remove the initial compression before they begin to experience a positive tension or stretch, while the expansion is continuously resisted by the tension of the outer layers.
6B3. Properties of gun steel
Before considering the construction of a gun according to this principle, it will be necessary to examine some of the properties of gun steel which have not yet been considered. Gun steel is elastic within limits: thus, if a stress is applied so as to set up a strain (deformation or change in dimension) not exceeding the elastic limit of strain of the steel, then the steel will return to its original shape and dimensions when the stress is removed. It is then said to have been worked within its elastic range. However, when the elastic limit of strain has been reached, if the stress is increased the steel will yield rather suddenly and suffer a comparatively large strain without further increase in stress. Thereafter increase in stress will still further increase strain. The steel is now being worked in its semiplastic range. (If the stress is still further increased the strain will go beyond the semiplastic range and the steel will give rapidly and fracture, even with decrease of load.) The important point is that the steel has now received a permanent set or deformation. Nevertheless, it will attempt to return to its former dimensions when the stress is removed. In other words, it has suffered a deformation that is permanent but elastic.
These properties of gun steel are plotted in figure 6B2, in which the ordinates, measured along OY, represent the stresses applied to a test piece, and the abscissas, measured along OX, represent the corresponding strains set up. The curve is drawn only to show tension stresses causing extension strain in the steel, but it could be shown that the steel behaves similarly under compression stresses causing compressive strains.
As the stress is raised from O to A, the steel is strained by the amount OC. If the load is increased slightly, the steel yields suddenly and suffers the additional strain CE at practically constant load. A further increase in the load to K causes an additional strain EG. The behavior of the steel thus far is represented by the curve OBDF.
If the load is now removed, the curve is seen to return, not to the origin but to the point H, the line FH being about parallel to OB. The steel has taken the permanent deformation, or strain, OH but still has elastic properties, as is shown by the decrease in strain from G to H upon removal of the load. HG is somewhat larger than OC. If the same test piece is again stressed, a stress equal to OK will be required to strain it by the amount HG; for purposes of such a second stress, H may be considered to be at the origin.
From the above, it may be seen that the steel has acquired two important new properties:
- It has received a permanent deformation, or strain, and will resist a compression stress tending to compress it to its former dimension (curve HM shows this action).
- It has changed its physical qualities in that the application of a stress beyond its original elastic limit, has given it a new elastic limit practically equal to the stress it has sustained.
Now consider the application of this principle to gun construction.
6B4. Built-up guns
In the simplest built-up gun we begin with an inner steel tube of outer diameter d, and place around it a cylindrical jacket of inner diameter d−s. s is small — on the order of 0.01 in.; s is called the shrinkage.
The usual method of doing this is to heat the jacket, thereby expanding it, and to slip it over the cold tube, allowing it to cool and shrink in place. The result is that the tube receives a strain in compression (negative extension), because of the shrinkage of the jacket upon it, while the jacket receives a strain in extension, being unable to shrink to its former size. These strains are well within the elastic limit of strain of the steel. We have here, then, not an initially unstrained steel, but a compound cylinder of two members, the inner of which has an initial strain in compression (negative) and the outer an initial strain in extension (positive).
When powder gas pressure (stress) is applied in the bore of such a compound cylinder, the pressure must first expand the tube enough to remove the initial strain of compression before it can continue the expansion toward the elastic limit of extension of the tube. Such expansion is continuously opposed by the jacket, which is pressing inward. This action may be stated in the following principle:
If any pressure be applied to a compound cylinder, the strain at each point will be the algebraic sum of the strain at the point before the pressure was applied and the strain which the same pressure could cause at the corresponding point in a simple cylinder, of the same dimensions as the compound one.
In a compound cylinder, according to this rule, the inner layer receives less strain in firing than would be received by the corresponding layer in a simple cylinder, for the original compression must first be overcome before any positive strain (extension) can be introduced. Correspondingly, the outer layer receives more strain than it would in a simple cylinder plus the original strain in extension that it receives in construction. The stress felt by the different layers of the gun is then no longer inversely proportional to the square of the radius according to Lamé's law, but instead is more evenly apportioned among the layers of metal.
This principle is applied in the built-up gun, which was briefly described in the preceding chapter, and is illustrated in figure 6B3. The principle of prestressing by shrinkage has its limits of application, however. Regardless of whether strains are set up by firing or by prestressing (shrinkage), the following limiting principle applies:
No fiber of any cylinder of a built-up gun must be strained beyond the elastic limit of the metal of that cylinder.
In a built-up gun, the outer cylinders, or hoops, are heated and assembled one at a time on the tube. As the hoops cool, they shrink, and tightly grip the cylinders within them. The locking rings are then added to prevent longitudinal movement of the hoops. After the hoops and locking rings are assembled on the tube, the entire assembly is heated and shrunk on a liner. The 16-inch gun is an example of this construction.
The liner, which carries the rifling, is usually thinner than the tube. It is, therefore, not to be considered a major strength member, since all but a small part of the strain is transmitted through the liner to the tube and hoops. The liner can be replaced when the rifling has worn down, without sacrificing the other parts of the barrel which have a much longer service life.
The assembled barrel forms a cylinder within which high pressure is developed as the charge explodes. The effect of the pressure is greatest on the inner cylinder, and diminishes rapidly as it proceeds outward. If the outer hoops were assembled over the tube without shrinkage, they would be subjected to less strain than the tube and the strength of gun would be little greater than the strength of the tube. However, the shrinkage of the hoops squeezes the tube, at the same time stretching the hoops. The safe pressure of explosion can then be increased, for it must overcome the squeeze before it can stretch the tube. The shrinkage is so calculated that each hoop carries a share of the strain.
6B5. Radially expanded guns
A gun made from a single cylinder which has been subjected to a radial-expansion process is called a radially expanded monobloc gun. In this process the gun forging is bored to a diameter somewhat less than the finished dimension, and turned down on the outside to something greater than its finished diameter. Hydraulic pressure is then applied to the bore. By Lamé's law the metal at various points through the wall of the gun will experience stresses which are inversely proportional to the square of their radii. The pressure in the bore is increased in steps, until a thin, indefinite layer of metal nearest the bore is brought to its elastic limit of strain. At this time all the other (imaginary) layers of metal in the forging are also strained, but all within their elastic limit, and the amount of strain decreases regularly as we consider layers of the metal more and more remote from the bore.
The pressure is now increased so that the bore layer is strained beyond its elastic limit, the layer next outside the bore layer is brought just to its elastic limit, and the tension in all the other layers is increased. Still further increase of pressure increases the permanent strain in the bore layer (which is now being worked in the portion of the curve BDF, fig. 6B2), strains the second layer beyond its elastic limit, brings a third layer up to its elastic limit, and increases the tension in all the other layers. The increase in pressure is continued until the outside layer of metal just reaches its elastic limit of strain, and this pressure is held for a time. This pressure is considerably greater than the pressure which the gun will be called upon to withstand when fired.
When the pressure is removed and the metal allowed to return to a state of rest, the physical condition of the forging is as follows:
- The bore layer, which has experienced the greatest stress and therefore received the greatest permanent strain, is pressing outward upon the second layer, for it tends to be larger than the second layer. Having received the greatest stress, it has a greater permanent-and-elastic limit than the second layer, and a greater elasticity.
- Conversely, the second layer, having received slightly less stress, is strained slightly less, has less elasticity, and is pressing inward upon the first layer.
- Continuing outward, the third layer bears the same relation to the second layer as the second does to the first, and so on.
The net result is that the inner layers are being pressed upon by the outer layers, and receive a strain in compression, as in the curve HM, figure 6B2, but they resist this pressing inward by pressing outward, and thereby place the outer layer in a state of tension. We then have a gun constructed by a process of self-hooping (autofrettage), made as if composed of an infinite number of infinitely thin hoops shrunk together and therefore demonstrating the principle of initial tensions.
The radial-expansion process results in a cylinder in which the change from squeeze on the inner layer to stretch of the outer layer is uniform. The change in a built-up gun is in steps from hoop to hoop, and the strength of the metal is not fully utilized. The metal in a radially expanded gun is used much more efficiently; therefore, radially expanded guns weigh less than built-up guns of the same strength. The reduced weight of the barrel makes possible a lighter gun assembly.
The radial-expansion process permits faster gun production at lower cost. With the saving in weight, this makes radially expanded guns preferable to built-up guns. However, the process is limited at present to moderate-size guns because of the difficulty of obtaining a single forging large enough for those of major caliber. Typical monobloc barrels are found in the 5"/38 caliber guns and the 6"/47 caliber guns.
6B6. Combination guns
The built-up and radially expanded methods may also be incorporated in a single gun. Thus the difficulty of obtaining a single forging big enough for the larger guns can be overcome. The 8"/55 caliber gun, for example, has a jacket shrunk on a radially expanded tube.
6B7. Simple one-piece guns
Many small guns such as the 40- and 20-mm are made from a single steel forging which requires neither radial expansion nor hoops. The pressures developed per square inch in small guns may be higher than those in large guns, but this may be compensated by increasing the size of the forging, which is not excessively large in any event. This type of construction is, at the present time, limited to guns of 3-inch caliber and smaller; but the development of steels with greater metallurgical strength may make it applicable to large guns in the future.
6B8. Rifling
Chapter 5 explained the nature and purpose of rifling. Figure 6B4 shows in a detailed cross section the chamber of a gun, a seated projectile, and the origin of rifling, and figure 6B5 shows details of gun rifling. The velocity of projectile rotation when it leaves the muzzle of a gun depends on the twist of the rifling and the velocity of the projectile. A 16"/50 projectile turns at about 4,000 rpm when it leaves the muzzle, and a 40-mm projectile turns at about 40,000 rpm.
In guns 5-inch and smaller, rifling is cut into the gun tube's bore. Larger guns may be fitted with tubular loose liners, which can be replaced with relative ease when the rifling is worn out. The rate of rifling wear tends to increase with caliber.
6B9. Differences in construction between case and bag guns
Nowadays only large guns (8-inch and up) use bag ammunition. Hence bag guns are generally of the built-up type, while a case gun may be monobloc or built-up, depending on size. Other differences in construction between case and bag guns are concerned only with the breech structure.
The breech end of a case gun generally terminates in an interrupted-screw thread which meshes with a similar thread in the gun housing. Figure 6B6 illustrates a typical arrangement, which is easy to recognize as an application of the interrupted-screw principle discussed in the preceding chapter. A key prevents rotation of the barrel with respect to the housing after engagement.
The breech end of a bag gun has a yoke, a massive metal ring, surrounding it. The yoke provides a connection between the barrel and other recoiling parts and the recoil and counterrecoil systems. Shoulders on the gun prevent movement with respect to the yoke. The yoke serves also as a counterweight to bring the gun's center of gravity toward the breech. The after end of the gun chamber contains the screw-box liner or screw box, a steel insert whose threaded interior surface meshes with the stepped thread of the breech plug. The liner is locked in position by keep screws, and can be replaced if worn or damaged. It is illustrated in the chapter on turrets.
6B10. Care of bore and chamber
Complete instructions for the regular inspection and cleaning of gun barrels will be found in the Bureau of Ordnance Manual and other publications of that bureau. Only a few of the more important aspects of gun maintenance will be discussed here.
Great heat, great pressure, and complicated chemical changes accompany the burning of the charge. Some but not all of the residue of the burning is blown out of the muzzle after the projectile. That which remains in the gun is in the form of a corrosive salt. Standard procedure is to remove this “fouling” by washing out the bore with a hot soda solution and applying a thin film of oil before securing until the next firing. Since the advent of chromium plating of gun bores, powder fouling is much less of a problem.
Dirt in a gun bore is not only an invitation to corrosion but a source of positive danger because, if it offers sufficient resistance to the passage of the projectile, excessive pressure may pile up at a point where the design of the gun will not withstand it. To guard against the accidental admission of dirt, spray, or moisture into the gun, a solid muzzle plug, rather like a cork, called a tompion (pronounced tom-kin), is inserted. This is only a partial solution, because under certain weather conditions considerable condensation accumulates in the bore. This moisture is also a source of corrosion danger. In fair, dry weather, tompions are removed to air out the barrels.
Tompions cannot be used under combat conditions, because of the possibility of one inadvertently remaining in a gun when firing. However, dirt and water, especially salt water, must be kept out of the gun; so canvas, or in the case of small calibers, plastic, muzzle covers are used. In an emergency, the projectile can be forced through such covers without bursting the barrel. This procedure is, of course, subject to certain limitations. Projectiles with supersensitive nose fuzes cannot be fired through muzzle covers of any sort. In cold-weather operations, when canvas covers may become ice coated, they should be removed before firing.
More immediately dangerous than corrosion or dirt is metallic constriction of the bore. Before and after each firing, barrels are tested for this condition with a plug gage, which is a steel cylinder accurately machined to slightly under the diameter of the bore. If at any time it is discovered that the plug gage will not pass through the bore without undue forcing, the nature of the constriction must be determined.
One type of constriction is coppering — consisting of metallic deposits on the bore, left behind by the rotating bands of projectiles. Even an amount of copper too slight to impede the projectile will affect its accuracy. Metallic lead foil in the powder charge, while increasing muzzle flash, has been used in some powders to control coppering. The lead, reduced either to a molten and thinly dispersed state or to a gaseous one, serves as a lubricant on top of which copper deposits will not form. Once so treated, new or increased deposits of copper will not occur, and the existing deposits will be abraded or swept along by the passing projectile. The firing of the older type of star shell, which has a lead gasket between its base and its flanged base plug, has a similar effect. The newest propellants have incorporated into their composition a trace of lead carbonate, much more readily reducible than even the finest metallic lead foil.
Copper fouling may also be removed with an acid treatment, but this is not authorized for shipboard use. Approved mechanical means for meeting this condition consist of rubbing away the constriction with a wire bore brush or with a lapping head such as shown in figure 6B7. The head is covered with a fine abrasive material and is drawn back and forth at the location of the constriction until the plug gage can be passed through without forcing. Special scraping or decoppering heads, fitted with steel blades, are supplied for certain guns.
Steel constriction also occurs in built-up guns. The friction of the projectile on the bore tends to drag the liner along with it, which tendency is resisted by the shoulders of the liner and the tube. With continued firing, the shoulders of the liner tend to override those of the tube, thereby forcing the walls of the liner inward. As with coppering, steel constriction can be removed by lapping and polishing.
Continued firing may also elongate the liner and cause it to protrude from the muzzle. This is not a serious condition. When the extension amounts to as much as half an inch, it is simply cut off.
C. Interior Ballistics
6C1. Ballistics
Ballistics is the science of the motion of projectiles. It is divided into two branches, interior and exterior ballistics. Interior ballistics is that branch of the science which treats of the motion of the projectile while in the gun. The initial velocity — i.e., the speed of the projectile at the time it leaves the muzzle of the gun — is a result of the various forces which are involved in the general term, interior ballistics. Exterior ballistics pertains to the projectile after it leaves the gun and will be considered in the fire control problem, discussed in Volume 2. Obviously the initial velocity is the one value common to both interior and exterior ballistics.
Gun design is essentially a compromise. The gunner naturally desires a maximum velocity for great range and flat trajectory; the designer must consider the strength of his gun and desires the minimum wear or erosion therein. The velocity finally agreed upon must take into consideration both of these requirements. To determine the velocity of the projectile at the muzzle of the gun requires a study of (1) the combustion of the powder, (2) the pressures developed within the gun, (3) the variations in pressures and velocities with changes in any of the “conditions of loading,” and (4) erosion at the bore. Such is the field of interior ballistics.
6C2. Propellants
Propelling charges are designed to burn in the chamber of the gun in such a way that the maximum velocity may be imparted to the projectile without excessive heat, pressure, or erosion. To accomplish this the thrust against the base of the projectile must be uniform. The most efficient propellant for a gun would be so balanced that the charge is entirely consumed immediately before the projectile leaves the muzzle.
A “high explosive” is one capable of instantaneous evolution of masses of highly heated gases. A “low explosive,” such as smokeless powder, is not detonated, but is burned in an appreciable length of time, causing a comparatively gradual evolution of gases, with consequently much less shock and wear to the container. From this may readily be seen the impracticability of using high explosives for propelling charges in guns, and the suitability therefor of smokeless powder. Figure 6C2 illustrates this fact.
A propelling charge must be suited to the gun in which it is to be used; that is, the speed of burning of the charge must, within close limits, be appropriate to the specific gun. Several factors are involved; for example, the size of the grain, the shape, the number of perforations, the web thickness between perforations, the percentage of nitration, the moisture content, the remaining volatiles, and the stabilizer used. Of these, grain size is the most easily changed, and it is varied to control the rate of burning. The percentage of nitration is fixed. The moisture content and the remaining volatiles vary with grain size. Diphenylamine stabilizer absorbs nitrous vapors, the first products of decomposition, the pressure of which would otherwise cause the generation of more vapors at a continually increasing rate.
The grain shape of gun propelling charges is normally cylindrical. The web thickness and the number of perforations vary with the size of the grain. For guns smaller than 40-mm the number of perforations is 1 or none, and for larger guns the number of perforations is usually 7. See figure 6C1.
A propellant's potential is defined as the total work that could be performed by the gases of combustion while expanding from the solid state to the space they would occupy when fully expanded to atmospheric pressure and when cooled to a specified temperature. It is of interest to note that there is less stored-up energy in smokeless powder than in most common fuels. The chief characteristic of an explosive lies in its enormous rate of delivery rather than in its amount of delivery. In the average conventional gun, some 60 percent of the potential disappears in muzzle loss; 30 percent is transmitted to the projectile, and all other losses — such as heating the projectile and gun, causing the gun to recoil, and so forth — amount to about 10 percent.
6C3. Gun strength-pressure relationship
To establish the basic principles of gun design, study figure 6C3. The figure may be taken as typical of the strength-pressure relationship in modern guns. Note that the high breech strength is carried well forward of the point of maximum pressure. The gun strength at every point must exceed the powder pressure at that point by an amount that will provide a suitable margin of safety.
The curve as it appears in figure 6C2 shows pressure beginning at a value well above zero. This indicates the pressure build-up that occurs after the propelling charge begins to burn but before the projectile begins to move. (The x-axis in the figure represents projectile movement in the bore, not time or bore length.) The projectile begins to move only after the propellant gas reaches the initial forcing pressure required to initiate movement of the projectile in spite of projectile inertia and the engagement of the rotating band in the rifling.
Note that the gun strength curve is represented as a straight horizontal line above the area between the point of initial forcing pressure and the point of maximum pressure. It does not vary in parallel with the pressure curve. The reason is that the same pressure that the expanding gases exert against the base of the projectile is exerted equally against all interior surfaces of the gun behind the projectile. Hence the breech part of the barrel must be designed for the maximum stress to be imposed.
After the projectile passes the point of maximum pressure, it continues to be accelerated by gas pressure until it leaves the muzzle. The total area under the curve, up to the point where the projectile leaves the gun, is a rough measure of initial velocity, and the pressure remaining at the muzzle is an indication of the muzzle loss. A high muzzle pressure increases muzzle flash.
6C4. Changes in “conditions of loading”
By “conditions of loading” are meant the powder used, the weight of charge, the density of loading, the volume and form of the powder chamber, and the weight of the projectile.
a. Powder used and weight of charge. Powders are spoken of as quick and slow powders, these terms being used only in reference to a particular gun. A slow powder is one in which the rate of combustion is comparatively slow, and a quick powder is one in which the rate of combustion is comparatively rapid. For instance, a small-grain powder is quicker than a larger grain of the same shape, since all the grains would be consumed in a shorter time. Not only will the larger grain increase the time required for burning the charge, but it will also cause maximum pressure to be lower and to be reached later in the travel of the projectile. The gun pressure curves shown in figure 6C4 compare slow powders and quick powders where the same weight of charge was used. Within limits, the muzzle velocity for a particular gun may be increased without causing excessive pressure by increasing the size of the charge and at the same time using a powder that burns more slowly. See figure 6C5.
b. Density of loading. Density of loading is the ratio of the weight of the charge of powder to that of the volume of water which, at standard temperature, would fill the powder chamber. It is a measure of the amount of space in which the gases of combustion may expand before the projectile begins to move. See figure 6C6.
It follows that a high density of loading leaves but little space for initial expansion, and consequently that the pressure builds up rapidly. Therefore the maximum pressure behind the projectile is reached early in the projectile's movement through the bore. With a lower density of loading, more expansion of the gases may take place before the projectile starts to move; the maximum pressure is achieved later, and this maximum is necessarily lower than that resulting from high density of loading. Other factors remaining equal, increased density of loading increases maximum pressure, muzzle velocity, and muzzle loss.
The densities of loading at present vary between 0.4 and 0.7, depending on the caliber of the gun and on whether the charge is case, stacked bag, or unstacked bag. Since the specific gravity of smokeless powder is about 1.6, the following relationship holds:
Density of loading = 1.6 v,
where v = the proportion of the total chamber volume which is filled by the charge.
Hence it is apparent that a loading density of 0.4 would require a charge filling 25 percent of the chamber volume, and a loading density of 0.7 would require a charge filling 45 percent of the chamber volume.
When the density of loading drops markedly below the above figures, irregularities of muzzle velocity may be expected. This is probably due to nonuniform ignition, excessive physical displacement of the powder grains during the burning, and an abnormal burning rate. Whatever the cause, it is evident that the pressure builds up irregularly instead of smoothly, and there is real danger that the high point will be reached at the wrong time.
A practical example of this would be a projectile lodged part-way down the bore of a gun, thus greatly increasing the effective chamber volume. Not only has this the effect of greatly lowering density, thereby causing pressure waves which may build up beyond a safe limit, but it also extends the area of maximum pressure beyond the area of maximum barrel thickness. Should a normal powder charge be used to dislodge a projectile so positioned, the result would be a burst, or at least a bulge, immediately behind the projectile.
Very high density of loading, on the other hand, may cause detonation of the propelling charge, again resulting in a burst gun.
c. Volume and form of powder chamber. The designers of the gun, having established first the desired muzzle velocity, then the limiting maximum pressure allowable in the gun (determined from study of gun construction), can proceed to determine the volume and form of the powder chamber and the weight of the charge. Once a particular gun has been built, the volume and form of the powder chamber changes only because of erosion at the origin of rifling and improper seating of the projectile. This will cause irregular muzzle velocity. Projectiles differing in weight — for example, high-capacity and armor-piercing types — can be fired from a given gun. High-capacity projectiles, being lighter, will have a slightly higher muzzle velocity.
6C5. Summary
The following conclusions may be drawn from the propellant pressure curves and the foregoing discussion:
- High explosives are not suitable for use as propellants.
- Using the same weight of charge, a slow powder produces a smaller maximum pressure than a fast powder, and attains this maximum pressure later in the travel of the projectile.
- Increasing the weight of a charge of powder of a given grain size increases the maximum pressure attained and causes this maximum to occur earlier in the travel of the projectile.
- Because of muzzle loss and irregularity of muzzle velocity, slow powders are less efficient than fast powders.
- The muzzle velocity of a given gun may be increased within limits by using larger charges of slower propellants.
- In general, powders designed for different classes of guns are not interchangeable.
D. Erosion
6D1. Causes of erosion
The deterioration and wearing away of the bore surface by use is known as erosion. This effect is not the direct result of friction caused by the projectile in its movement down the bore. There is some uncertainty about the exact process by which the interior of a gun wears away, but it is generally agreed that the following are the principal causes:
- The inner surface becomes intensely heated in firing, and the rush of hot gases across this hot metal has a scouring effect.
- The hot powder gases react with the metal, changing the carbon content on the surface of the bore. Since this surface is designed with an optimum carbon content, any change results in a weakening of the metal.
- The alternation of intense heat and rapid cooling affects the temper of the metal.
- The explosion gases are forced into and out of the pores in the metal surface as they open and close during the expansion and contraction which accompanies such drastic temperature changes.
- Heat cracks may develop.
- Gases escaping around the projectile act as high-velocity jets, scouring the bore and causing damage, especially where there are heat cracks.
6D2. Effects of erosion
Two fundamental facts of erosion are (1) that it is always greatest at the origin of rifling, and (2) that the tops of the lands wear away faster than do the bottoms of the grooves.
Enlargement at the origin of rifling, in bag guns and guns using semifixed ammunition, tends to permit the projectile to seat farther and farther toward the muzzle. This reduces the density of loading and therefore the muzzle velocity. To avoid this, there is a lip of slightly greater diameter at the base of the rotating band which tends to engage the forcing cone more nearly at the same point at every loading, regardless of gun wear. Naturally, this applies only to bag and semifixed ammunition, where the projectile is not positioned by the case. In all guns, however, erosion at the origin of rifling permits gas to escape around the projectile, and this in turn increases erosion.
As the lands wear, not only does more gas escape around the projectile but the rifling engraves the band less deeply, reducing materially both the initial forcing pressure and the resistance of the projectile to the gas pressure. The effect is a material drop in muzzle velocity.
6D3. Control of erosion
All erosion factors are related to (1) the temperature of the expanding gases and (2) the duration of their confinement in the bore. So, larger guns, with their slower powders and longer barrels, suffer more erosion per round fired than smaller guns, but the higher firing rate of the lesser guns offsets this, because it permits less cooling time between rounds.
Chromium plating of gun bores has reduced the effects of erosion, and it may be possible that in the future the use of molybdenum in this fashion will make for even better erosion resistance.
Some smaller guns are cooled by water jackets around the barrels, and experiments are under way on introducing a coolant between the tubes and the liners of larger guns. Also under development are cooler propellants. Any development that reduces the heat of explosion will aid in erosion control and, inasmuch as excessive erosion imposes much of the present limitation upon muzzle velocity, temperature reduction appears to be a most practical approach to more effective gunnery.
6D4. Gun life
Erosion, however carefully controlled, eventually terminates gun life. Though, with the exception of the larger turret installations, regunning is relatively simple, it cannot be done under combat conditions. Therefore, the duration of the effectiveness of a gun is of paramount importance.
Symptoms of the end of serviceability are: (1) loss of accuracy, (2) loss of velocity, and (3) erratic fuze actions. Naturally, a gun that does not hit its targets is no longer useful; neither is one of inadequate range and armor-piercing capacity, both of which result from velocity loss. Erratic fuze action not only undermines effectiveness but can endanger friendly personnel.
Any of these conditions can take place when the rifling no longer imparts adequate rotational velocity to stabilize the projectile in flight. In small arms, this effect can be seen when tracers are used, and a barrel should be replaced when erratic flight is observed. With larger guns, however, it is important that replacements be made before the effectiveness is seriously reduced.
Up to the present, relining guns has been a naval shipyard operation, but recent experiments indicate that it may soon be possible to use a loose liner with as much as 0.010 inch between it and the tube. This will make it possible to regun aboard ship under active service conditions. With expendable liners, much higher muzzle velocities with their increased erosion would become acceptable.
6D5. Erosion measurement
There are, for each class of gun, curves furnished to ships showing relationship between the enlargement of the bore and the initial velocity to be expected from the gun. Thus if the actual diameter is frequently checked, velocity loss becomes predictable, proper allowance for it can be made in aiming, and, of at least equal importance, barrels or liners may be replaced before their performance becomes noticeably erratic.
In some minor-caliber guns, measurement of bore enlargement is made at the origin of rifling only. This is done with a wear gage, which is a truncated cone that can be inserted directly in the breech. In larger guns, erosion is measured at several points in the bore with a star gage.
The star gage is a simple device, consisting basically of a hollow staff with a head at one end and a handle at the other. By means of the staff, the head can be inserted in the bore to the desired point. Three removable points, the length of which varies with the caliber of the gun to be measured, are carried in sockets spaced radially 120° apart in the head. The sockets are pressed inward upon a cone by spiral springs and move inward or outward, at right angles to the staff, as the cone, activated by a threaded rod which the hollow staff contains, is advanced or retracted. A vernier on the handle end of the staff measures this inward and outward movement of the points in thousandths of an inch.
Before use, the star gage is calibrated by inserting the points into a standard ring, accurately machined to the designed bore diameter of the gun, and setting the vernier at zero. Then, upon insertion into the bore, vernier readings directly measure bore enlargement. As a rule, two readings, 180 degrees apart, are taken at each point of the bore to be measured.
Of these measurements, that taken immediately forward of the origin of rifling is the most important. It has been found that muzzle velocity loss is a function of bore enlargement at this point, and tables or graphs, such as those illustrated in Volume 2, are predicated upon it.
Specimen curves (for 5"/38 guns Mk 12 and mods) used in figuring equivalent service rounds and velocity loss are illustrated in appendix B. One curve shows equivalent service rounds plotted against measured bore enlargement (in thousandths of an inch) at the origin of rifling. The other curve shows directly velocity loss (from the nominal standard service value of 2,600 fps) as plotted against bore enlargement at the same point. Thus, from the measurement of bore enlargement, it is possible with these curves to determine equivalent service rounds fired, and velocity loss in feet per second.
6D6. Velocity loss estimation
Star gages are not carried on combatant ships, and star gaging is done only when shore- or tender-based facilities are available, however desirable it might be in theory to do it before and after every firing. After each firing, when a star gage is not available, the additional bore enlargement must be approximated.
Estimates depend upon the fact that, under normal conditions, each round fired causes a certain amount of erosion, experimentally determined for various periods in the life of the barrel. The standard unit of measurement is equivalent service rounds (E.S.R.). Reduced charges, which are used for such purposes as gunnery practice, have less effect than service charges. Increased charges, used for proving guns, have more effect on bore enlargement. When such charges are fired, their effect must be reduced to E.S.R.'s before the curves can be used. For instance, it has been determined that a reduced 1,200 fps charge will cause only one-sixth the erosion of a full 2,600 fps charge. Consequently, for computation purposes, six such rounds would be regarded as one E.S.R.
The legend accompanying the graphs gives the proper method of using them. (See appendix B.) Other corrections are required when firing certain indexes of powder, but such information is included in the same Bureau of Ordnance publications as the curves.
Without periodic star gaging, the equivalent-rounds procedure would not be accurate, mainly because the typical curves take no account of the rate of fire. Rounds fired with very short cooling intervals between cause much greater erosion than the same number of rounds fired at normal intervals. For practical gunnery, however, the curves are considered sufficiently reliable for use, if no better data can be obtained.
6D7. Improved methods of measuring I.V.
At proving grounds gun projectile velocity is measured by a device called a chronograph. In one (older) type the projectile cuts wires when it passes through two successive screens located in the trajectory at known ranges, and the exact time of each passage is recorded. This yields the projectile velocity between the screens, and from this I.V. can be reckoned. In a more recent type, the projectile passes through two magnetic coils in succession, and the induced impulses, amplified, cause sparks to pass through a rapidly moving tape. From this the elapsed time and then the I.V. can be worked out.
Both of these methods require special setups of coils or screens, careful gun placement and aim, much auxiliary equipment, specially skilled technicians, and a good deal of time. They are consequently not practical for shipboard use. But because erosion and I.V. are significant factors in fire control, the Navy has developed chronographs on far different principles which can actually measure service rounds fired on shipboard. One such design incorporates the velocity-measuring device in a fire control system. In the other, the chronograph device (which works on a different principle from those already mentioned, since it measures projectile velocity in the gun bore) is mounted in the barrel and on the carriage as an independent unit.
E. New Developments in Gun Design
6E1. Attainment of high muzzle velocity
Defense against modern high-speed aircraft is largely a fire control problem. Firing is at a predicted future position of the target, and the less time the projectile spends in the air between the muzzle and the objective, the less time hostile aircraft have for defensive maneuvering. Higher muzzle velocities are not the whole answer, but do contribute to the simplification of the problem.
Of the several means of achieving higher muzzle velocities, the following either were used in action in World War II or were experimented with. See figure 6E1.
Lightweight projectiles fired from conventional guns attained muzzle velocity of about 4,000 feet per second as compared with 2,700 for the standard projectiles. Air resistance, however, so quickly retarded this type of projectile that it was of slight use at the longer ranges prevailing in naval warfare. It was used with great effect ashore in antitank warfare, where ranges were shorter.
Subcaliber projectiles fired from conventional guns lightened the projectile and, at the same time, presented less surface for air resistance. This is a more slender projectile fitted with a lightweight bushing called a sabot which fits the gun bore and drops away after the projectile leaves the muzzle. The German army achieved muzzle velocities above 5,000 fps with guns as large as 11-inch and also fired sabot projectiles from 90-mm antiaircraft guns. This too has disadvantages for sea use, as the discarded sabot is dangerous to nearby friendly ships.
Conventional projectiles fired from high-strength or extra long guns can attain a velocity limited only by the amount of pressure that the gun can withstand and by the amount of pressure that can be developed with available types of powder. Such guns would be heavier and require considerably more space aboard ship than guns which impart conventional velocities to standard projectiles.
Tapered-bore guns firing “skirted” projectiles have many of the advantages of the sabot without the danger to friendly personnel. This subcaliber projectile is equipped with flanges which furnish a gas seal and which are squeezed inward by a tapered reduction in the size of the bore toward the muzzle. There is some loss of accuracy and a smaller projectile is fired, but these effects are more than compensated for by the reduced time of flight. The Germans used such “squeeze-bore” guns in sizes from 20- to 75-mm and attained 4,700 fps muzzle velocities with them.
Rocket-assisted projectiles were used by the Germans in large guns, the rocket action being initiated during flight. The range of this projectile was greatly increased, but the space in the projectile occupied by the rocket propellant reduced the payload of high explosive that the projectile could carry. It appears doubtful that rocket-assisted projectiles could consistently provide the accuracy obtainable from other projectiles, but they are worthy of consideration for firing at very long ranges.
6E2. Design aspects of high-performance gun barrels
Ordinary-performance guns have always been made with a high safety margin, but the development of the high-performance barrel, with its concomitant bulk and other complications, required a re-study of gun design, so as to cut safety margin to a minimum. It was with some surprise, during this study, that ordnance engineers learned that the projectile set up higher stresses within the gun than the powder pressure. The liner is obliged to stretch to allow the projectile to pass through, and to resume its normal diameter after the projectile has passed, rather like an ostrich swallowing an orange. See figure 6E2. Entirely new methods of stress analysis had to be developed to evaluate the influence of this previously unrecognized factor.
6E3. Disadvantages of high-velocity guns
Reduced time of flight increases a gun's effectiveness, but, especially for shipboard use, there are other considerations. Such guns must necessarily be larger and heavier than conventional guns, so much so that the installation of one high-velocity gun on a present day naval craft might mean sacrificing several guns of ordinary velocity. Moreover, because of the additional powder required, ammunition handling would be complicated and it would be harder to maintain a high rate of fire. Further, these guns wear out more quickly than those designed for ordinary-velocity projectiles. A careful evaluation of all advantages and disadvantages of various types of guns is necessary in determining optimum armament of a ship.