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                                                                 D. Projectiles
3D1. General

The projectile is that part of a round of gun ammunition which is expelled from the gun by the force of the explosion of the propelling charge. Present-day projectiles are elongated cylinders with a pointed front end. The application of the principle of rifling to guns caused the abandonment of the earlier spherical projectile. Rotation of the projectile permitted the use of a longer and heavier projectile, thus obtaining vastly increased range, accuracy, penetrative ability, and sectional density. (See art. 3D5.)

Modern small-arms projectiles often consist of solid metal; projectiles used in larger guns, however, are assemblies of several components. The three essential parts are the metallic body, the explosive bursting charge, and the fuze which sets off that charge. There may also be a tracer to make the projectile more readily visible during flight. Fuzes and tracers will be discussed in the next section of this chapter.

3D2. Projectile bodies

The solid bullet damages by impact alone. Assembled projectiles, however, inflict damage primarily by the blast of the high-explosive charge and the resulting high-velocity fragments.

The external shape of the projectile is designed to obtain the desired flight characteristics of stability and minimum air resistance. The form of forward end which best fulfills these conditions is the ogive. An ogive (fig. 3D1) is the shape generated by the revolution of an arc of a circle about a chord. In a projectile the chord is the axis of the projectile and the radius used is about nine times the diameter (caliber) of the projectile. (In small-caliber projectiles a cone is sometimes used instead of an ogive.) Abaft the ogive a projectile is cylindrical. The cylindrical shape may continue to the base, in which case the projectile is said to have a square base; or the after portion may be slightly tapered or conical, in which case the projectiles are described as boat-tailed. The corner of the base in either type is usually turned to a small radius. In fixed ammunition the form of the after end is influenced by the need for providing a bearing surface for the lip of the cartridge case.

Between the two ends lies the cylindrical body of the projectile. Near the after end of the cylindrical part of the projectile is the rotating band; at the forward end is the bourrelet. These two surfaces, slightly raised above the body, provide the support and bearing which steady the projectile in its passage through the gun. They must be some distance apart to prevent excessive wobbling in the bore. Except for the bourrelet, the projectile does not require a fine machined finish; experimental firings have indicated that fine body finish adds very little to projectile accuracy. See figure 3D2.
3D3. The bourrelet

The forward bearing surface of a projectile is machined to a fine finish to reduce friction and minimize the wear of the gun. In small projectiles the entire body forward of the rotating band may be finished to bourrelet diameter. On large-caliber projectiles additional bourrelets, abaft and forward of the rotating band, are added to provide better support, especially during ejection from the muzzle. A certain clearance must be provided between the bourrelet and the lands (the raised portions of the rifling). Standard United States Navy practice requires a specified bourrelet diameter 0.015 to 0.023 inch (in different-caliber projectiles) smaller than the diameter of the bore. To this margin is added a manufacturing tolerance of minus 0.005 to 0.007 inch, so that total clearance limits vary from 0.015 to 0.030 inch. Unnecessary clearance adversely affects accuracy and fuze performance and may mar the rifling by excessive wobble.

3D4. Rotating band

The three primary functions of the rotating band are to seal the bore, to position and center the rear end of the projectile, and to impart rotation to the projectile. Its secondary function is to hold the projectile in its proper position in the gun after loading and ramming, and to ensure that it will not slip back when the gun is elevated. The band has considerable effect on muzzle velocity, range, accuracy, and the life of the gun.

Rotating bands are usually made of fine copper; in major-caliber projectiles a small percentage of nickel is added to provide greater strength. Some projectiles of recent design have been banded with gilding metal (90 percent copper, 10 percent zinc), which increases strength and reduces the amount of copper deposited in the bore of the gun.

To reduce dependency on copper for this use (copper is increasing in military importance while it becomes scarcer and more expensive) rotating bands of sintered iron are under development.

United States Navy projectiles generally have rotating bands about one-third caliber in width. Foreign services sometimes use narrow multiple bands on major-caliber projectiles. The rough band is assembled (after heating it, in 8-inch and larger calibers) by slipping it over the rear of the projectile and pressing it into a score cut into the body of the projectile. This scoring usually includes a dovetail on each edge to assure that the band will not be thrown off by centrifugal force. Either waved ridges, longitudinal nicks, or knurling are provided on the bottom of the score to ensure against band slippage during rotary acceleration.

The forward edge of the band is slightly conical, to facilitate engagement with the origin of rifling. The cone, during loading, wedges into a seat at the origin of the rifling (except in fixed ammunition) and holds the projectile in place during loading and elevating. The central portion of the band is cylindrical and of a slightly greater diameter than that of the bore plus the depth of the rifling. This portion is sometimes divided by circumferential grooves, called cannelures, which provide space into which displaced copper may be wiped. In the after part of the band separate-loading projectiles have a raised lip followed by an especially deep cannelure. The lip serves to ensure a good gas check and also to prevent overramming in a badly worn or eroded gun.

The purpose of the cannelures is to minimize the formation of a fringe or skirt from the excess metal which is wiped rearward. Such a fringe is likely to flare outward, at the muzzle of the gun, due to the effects of the gases and of centrifugal force, and cause loss of range and accuracy. Bands on which the lip is well forward of the end of the band and is undercut with a deep cannelure are known as nonfringing.

3D5. Weight of projectiles

Within reasonable limits, a gun can fire projectiles of varying weights. Approximate weights of United States Navy projectiles are determined by the formula

W = d cubed/2

in which:

W=weight of projectile in pounds, and
d=caliber of gun in inches.

The weight of the projectile per square inch of bore is called sectional density. It is represented by the expression

SD = W/A

in which:

SD= sectional density.

W = weight of projectile in pounds.

A = area of bore, including grooves, in square inches.

This ratio varies with the size of the gun, averaging approximately six-tenths of the caliber. The concept of sectional density helps the designer to avoid designing a projectile of given diameter and weight either too long or too short for proper stability.

The distribution of weight in a projectile is a matter of considerable importance. The center of gravity should be in the longitudinal axis and close to or abaft the center of form.

3D6. Classification of projectiles

All gun projectiles, other than small arms, share the characteristics thus far described, but since targets differ in character, projectiles must differ in design, the better to defeat them.

The primary classification is into three general types:

1. Penetrating.

2. Fragmenting.

3. Special-purpose.
3D7. Penetrating projectiles

This type includes armor-piercing (AP) and common (Com). They are designed to penetrate, respectively, heavy and light armor. The usual bursting charge for these types is Explosive D, which is insensitive enough to permit penetration without premature detonation. The characteristics which make that possible will be described under the heading of penetration in the next chapter.

3D8. Fragmenting projectiles

These projectiles are designed to inflict damage both by blast effect and by fragmentation; that is, breaking up into small high-velocity fragments. They are characterized by thin walls and large cavities for the explosive filled. The general type is subdivided as follows:

1. High-capacity (HC) projectiles (fig. 3D3) are used against unarmored surface targets, shore objectives, or personnel. Since no penetration ability is required, explosives more sensitive than Explosive D may be used.
2. Antiaircraft (AA) projectiles are designed for use against airplanes in flight. Except for fusing they are substantially the same as high-capacity in the larger calibers. In smaller sizes the explosive often contains an incendiary element.

3. Antiaircraft common (AAC) projectiles are a dual-purpose design, combining the qualities of antiaircraft projectiles with the toughness necessary to penetrate steel plating not of armor thickness. The type of fuzing will depend on the use. The walls may be heavier than those of the other thin-walled types, and the filler is usually Explosive D.
3D9. Special-purpose projectiles

These are not intended to inflict damage by explosion or by fragmentation. Their construction incorporates no strength other than that required to withstand discharge from the gun without damage to the contents. If the filler includes any explosive, it is a small charge designed to expel the contents of the projectile. See figure 3D4. Some of the common varieties are:

1. Illuminating (Illum) projectiles, often called star shells (SS), contain a bright flare attached to a parachute. They are expelled from the projectile body by a small black-powder charge, which also lights the flare. As the parachute slowly lowers the flare, it serves to illuminate the target.

2. Smoke, or white phosphorus (WP), projectiles contain tubes of that substance which are scattered and burst by a small black-powder charge. White phosphorus produces a screening smoke. It also has some incendiary effect.

3. Window (W) projectiles contain metal foil strips, which, when scattered high in the air by the small burster charge, serve to confuse enemy radar operators.

4. Non fragmenting projectiles are used for antiaircraft gun practices. They contain a smoke-producing substance, available in various colors, which makes it possible to observe the bursts without close bursts destroying the target.

5. Target or blind-loaded (BL) projectiles contain an inert substance, often sand, designed to give the same weight and balance characteristics as explosive fillers. In large calibers (6-inch and above) target projectiles simulate the AP design but have no filler other than the spotting color.

6. Proof-shot projectiles are used for proof tests of guns at the proving ground.

3D10. Dye loads

Penetrating projectiles designed primarily for use against surface targets usually contain small quantities of dye, so placed in the nose of the projectile as to be dispersed upon water impact. This dye colors the splash produced by the hit and thus allows a ship to identify its own splashes. Standard practice is to issue to each ship in a division its own identifying color. Available colors are red, blue, green, and orange.

3D11. Projectile markings

Projectiles are painted various colors to facilitate rapid identification by gun crews. Nose fuzes and rotating bands are never covered with paint. Bourrelets are covered with one thin coat of paint only, and may never be repainted or retouched. The remainder of the projectile is painted according to the code set forth in figure 3D5, which applies to all calibers larger than 40-mm. Separate special codes are used for painting 20- and 40-mm projectiles.