Guns and Interior Ballistics




What You Should Know About Guns and Interior Ballistics

As a Fire Control Technician you will not be concerned directly with your ship's armament. You will not be called upon to maintain or repair the guns themselves, and only under emergency conditions will you be expected to operate them. Operation, maintenance and repair of your ship's guns is the job of the Gunner's Mate. However, a ship's armament would be useless without an effective fire control system, and vice versa, so you should know something about how and why the guns on your ship work.

Fire control the ships guns...make a team.

Two very important elements of the fire control problem are a projectile's initial speed and direction. You are interested, therefore, in gunpowder, guns and interior ballistics to the extent that you should become familiar with the factors which give a projectile its initial speed and direction. The purpose of this section of instruction sheets is to tell you all you need to know about guns and interior ballistics.

You are not expected to be a ballistics expert or an ordnance designer after you finish reading this section, for these subjects are major studies in themselves. A general understanding of basic theory, gun construction and familiarization with the terminology is all that is required.




Gun Construction

A gun is a tube capable of containing and controlling the explosion of a propelling gunpowder charge so as to discharge the projectile at a high velocity. The caliber of a gun is the inside diameter of its bore in inches or millimeters.

Length of boare and chamber = 150 inches, bore is 3 inches, 3 inch/50 caliber

All guns of 3-inch caliber and above have their length expressed in calibers immediately following the bore designation-a 3"/50 caliber gun has a 3-inch bore and is 50 calibers (150 inches) long. Guns used aboard Navy ships range in size from 16"/50 caliber guns on battleships, which can fire a projectile more than fifteen miles, to 20-mm automatic antiaircraft guns used aboard all Navy ships. Though these guns vary greatly in size and operation, their basic components are essentially the same. The major parts of a gun are illustrated below.

Major components of a 16 inch gun.  Barrel, Yoke, recoil mechanism, breach.

The breech is the rear end of the gun into which the projectile and propelling charge are loaded. The barrel is the tube through which the projectile travels after the propelling charge is fired and which gives the projectile its initial direction. The yoke connects the barrel and breech to the recoil mechanism. Part of the recoil mechanism may be mounted on the gun, but the recoil mechanism is primarily a part of the gun mount which will be discussed later.

Guns are classified according to bore diameters: (1) major-caliber, 8 inches or larger; (2) intermediate-caliber, greater than 4 inches and less than 8 inches; (3) minor-caliber, greater than 0.60 inch but not more than 4 inches; (4) small arms, 0.60 inch or smaller. We will not discuss small arms in these sheets because their fire control systems are simply sharp eyes and steady hands.




Gun Construction (continued)

The bore of a gun is the interior of the barrel. It is uniform in diameter and extends from the chamber (which receives the projectile and propelling charge during loading) to the muzzle of the gun.

Cross-section of a gun barrel.  Chamber, barrel, bore, muzzle.

The bore is "rifled," which means that a series of twisted grooves are cut on its surface. The rifling imparts a spin to the projectile as it travels through the bore, and this spinning motion prevents the projectile from "tumbling" after it leaves the muzzle. You will learn more about this "tumbling" action when you study exterior ballistics.

The effect of a rifle bore.  Rifled bore spins projectile.

In order for a gun to discharge a projectile at high velocity, the force resulting from the explosion of the propelling charge must be exerted on the rear of the projectile. To accomplish this, guns are equipped with breech mechanisms which seal the breech of the gun during firing but which can be opened to allow reloading.

Firing, breach closed.  Loading, breech open.

Breech mechanisms may be very simple as on a single-shot rifle, or very complex as on a 18-inch gun, but they always serve the function of sealing the rear end of the gun tube during firing.




Gun Mounts and Turrets

Guns must be mounted aboard ship in such a manner that they can be rotated horizontally (trained) and vertically (elevated). By means of these two motions a gun can be pointed in any direction.

These two motion can point the gun in any direction.  Elevation, Train (Traverse)

The mechanism which supports the gun and moves it in elevation and train is called the gun mount. The major components of a typical gun mount are the elevating mechanism, the traversing mechanism, the recoil mechanism, the trunnions, the carriage and the stand.

Major Gun Mount Components.  Elevating mechanism, traversing mechanism, stand, carriage, recoil mechanism (at rear).

The elevating mechanism may be power or hand driven and moves the gun in elevation; the traversing mechanism, also power or manually operated, traverses (trains) the weapon. The recoil mechanism absorbs the forces resulting from the explosion of the propelling charge and allows the gun to recoil (move to the rear). The stand supports the entire gun and mount and is rigidly attached to the deck; the carriage rests and rotates on the stand so that the weapon can be traversed. The trunnions provide a pivot support between gun and carriage so that the gun can be elevated.




Gun Mounts and Turrets (continued)

Most major-caliber guns (8" and up) are mounted in heavily armored structures called "turrets." Intermediate-caliber guns (over 4" and less than 8") are mounted in unarmored gun houses, or are provided with shields; minor-caliber guns (over 0.60" to 4") are shielded or simply mounted in the open.

A turret performs the same functions and has the same major components as the gun mounts discussed on the previous sheet. A turret differs from an ordinary mount in that it is heavily armored and fully enclosed. Two or three guns of 8-inch caliber or larger are mounted in a single turret.

The major components of a gun turret are shown below, with the names of the equivalent mount parts indicated.

Turret components. Barbette, gun house, gun girder (carriage), deck lug (trunnion), rollers, turret foundation (stand)

The rollers allow the gun girder (carriage) to traverse on the turret foundation (stand). The gun house provides armor protection for the gun crew, and the heavy armor of the barbette protects the lower turret components.

Gun mounts and turrets differ widely in complexity and type, depending on the size, type and number of guns mounted. It is hard to compare the free-swinging mount of the 20-mm antiaircraft gun (which has no elevating or traversing mechanisms) to the massive and intricate 16-inch gun turret, but they all serve the same basic function-to position the gun in accordance with fire control directions so that the projectile will hit the target.




Characteristics of Explosives

In order for a projectile to be fired from a weapon it is necessary that a force be exerted on the rear of the projectile. The stretched rubber band of a slingshot supplies the propelling force to a stone; the taut string of a bow exerts the force necessary to launch an arrow toward the target. In a similar way, an explosion (rapid burning) of gunpowder in a gun generates hot gases which expand and exert a tremendous force on the rear of the projectile, to propel it out of the gun.

Propelling forces.. Taut bowstring, stretched rubber band, burning gunpowder.

"Gunpowder" is a term applied to a family of explosive substances which when ignited burn very rapidly, liberating large volumes of high-temperature gases which create very high pressures. If this pressure is confined in a gun barrel, it will exert a great force on a projectile contained in the tube, forcing the projectile out of the gun at high velocity. When the projectile hits the target, the explosive within the projectile is set off. The resulting explosion and damage to enemy ships or aircraft is the only reason for carrying guns and fire control equipment aboard Navy ships. There are two general types of explosives, called "low" explosives, to fire the projectile from the gun, and "high" explosives, used as bursting charges within projectiles (or mines, torpedoes, etc) to destroy or damage the target.

Types of explosives.. Low explosive, propelling charge.  High explosive, bursting charge.




Characteristics of Explosives (continued)

The difference between low and high explosives is primarily in the rate at which the explosive burns. A low explosive has a relatively slow burning rate, and thus generates pressure within a confined space more slowly than does a high explosive which reacts almost instantaneously. A high explosive is desirable for inflicting a maximum amount of damage on the enemy but if used as a propelling charge would probably destroy the gun in which it was used. The reason for this is shown in the graph below which indicates the pressure generated within a gun barrel by both types of explosive as the projectile is fired. The high explosive would create an initial pressure which would exceed that which the gun can withstand.

Characteristics of high and low explosives.

Both high and low explosives require the application of some form of energy to initiate their explosive reaction. This initiation is supplied by heat for low explosives and by shock (detonation) for high explosives. This initiating energy is supplied by a small charge of "initiating explosive," a very sensitive high explosive which produces both a flame and shock when set off by being struck sharply.




Ammunition and Projectiles

Ammunition is the name given to the group of components which make up a complete charge or round for a gun. These components are (1) the primer, (2) the propelling charge, (3) the projectile, and (4) the fuse.

A round of fixed ammunition for a 3-inch automatic gun.

The primer contains a small charge of initiating explosive which, when activated by the firing mechanism of the gun, sets off the propelling charge and fires the gun. The projectile is the "pay-off"-the missile which is fired at the target. The fuze is fastened to the projectile and sets off the high explosive projectile charge when the projectile strikes or approaches the target, depending on the type of fuze used.

Ammunition may be of the fixed, semi-fixed or bag type. Fixed ammunition has all of its components assembled in one unit with the primer and propelling charge contained in a cartridge case which is attached to the projectile. This type of round is illustrated above and is used for all automatic guns, from 20-mm through 3-inch. It is necessary that automatic guns use fixed ammunition in order that they may be loaded rapidly by means of automatic equipment. After each round is fired the empty cartridge case is automatically ejected from the gun.




Ammunition and Projectiles (continued)

In semi-fixed ammunition the primer and propelling charge are contained in one unit and the projectile and fuse make a separate unit. This type of ammunition is used in 5-inch, 6-inch and some 8-inch guns. The need for loading each of the two units separately does not make this type of ammunition suitable for automatic weapons.

A round of semi-fixed ammunition for a 5-inch gun.

The third type of ammunition, which is used in some 8-inch guns and all guns larger than 8 inches, is bag ammunition. In this case the primer is separate from the propelling charge so that the round is loaded as three separate units. The primer is inserted into the firing mechanism of the gun rather than being built into the case of the propelling charge.

A round of bag ammunition for a 16-inch gun.

The propelling charge of bag ammunition is contained in raw silk bags which burn completely during firing and leave no residue. The number of bags of propelling charge used to fire a projectile depends on the particular gun used and range desired.




Ammunition and Projectiles (continued)

The projectiles used in a given gun vary considerably depending on the purpose for which they are used. Shown below are five different types of projectiles, each designed to serve a particular function. Several elements of the various types, however, are common to all. The rotating band which engages the rifling of the bore as the projectile travels through the gun tube, keeps the propelling gases from escaping past the projectile, and gives the projectile a spinning motion is common to all types; the projectile body in all types serves as a base to which all other projectile elements are attached.

Projectile types.  Armor piercing, dummy, common antiaircraft, common, illuminating.

The common antiaircraft projectile is used against air targets, and uses either a time fuse (which explodes the charge a pre-set length of time after firing) or a proximity fuse (which functions when the projectile comes within a certain distance of the target). The common projectile uses a windshield to reduce air resistance during flight and is used against surface targets. The illuminating projectile uses a time fuse and contains an illuminant which gives off intense light when the projectile bursts, illuminating a large area. The armor piercing projectile has a hardened steel cap to penetrate armor by impact before exploding. The target projectile has no explosive or fuse and is used only for target practice. Tracers, which burn during flight so that the gunner can observe the flight of the projectile, are provided in the base of many projectiles.




The Meaning of Interior Ballistics

The term "ballistics" was derived from "ballista, " which was the name applied to an ancient weapon used for hurling large stones or other missiles against fortifications. Today ballistics means the study of the motion of a projectile from the time the propelling charge is ignited until the projectile hits the target. "Interior ballistics" is the study of what happens while the projectile travels through the bore of the gun-from the instant of firing until it leaves the muzzle of the gun. "Exterior ballistics" refers to the action of the projectile during its flight from gun muzzle to target and to the factors affecting this travel. As a Fire Control Technician you will be mainly concerned with the problems involved in the study of exterior ballistics.

Scope of interior and exterior ballistics.  Inside the gun vs. outside the gun.

At the instant a projectile leaves the muzzle of a gun it possesses two properties: initial direction and initial velocity. The initial direction is controlled by the gun's elevating and traversing mechanisms; the initial velocity is determined by the interior ballistics of the gun. On the following sheets you will learn about the factors which affect a projectile's initial velocity; during your study of exterior ballistics a little later you will learn how initial velocity affects projectile range.

Interior ballistics as it affects a projectile's velocity includes the study of (1) combustion of the propelling charge, (2) gas pressures developed within the gun, (3) the speed of the projectile at various points within the gun bore, and (4) erosion (wearing down) of the gun bore.




Factors Affecting Projectile Initial Velocity

One of the most important factors of interior ballistics affecting a projectile's I. V. (initial velocity) is the rate at which the propelling charge burns. You have already learned the difference between low and high explosives, and why low explosives are used for propelling charges. Now let's compare different types of low explosive propelling charges and see how the rate of burning affects projectile I. V.

All Navy guns use propelling charges consisting of cylindrical grains, perforated from end to end with one or more holes. This type of charge results in the least muzzle loss (explosive energy wasted after the projectile leaves the muzzle) and highest muzzle (or initial) velocity for a given charge. A comparison is made below of the effects of different propelling charges (consisting of various powder grains) upon the velocity of the projectile in the gun bore. The larger, solid powder grains burn slowest, resulting in greatest muzzle loss and subsequent lowest I. V. The most efficient is the small perforated grain, the fastest burning of the low explosives.

How rate of burning affects I.V.

Thus you see that greatest muzzle velocity (I. V.) is obtained by using small perforated grains which have a higher rate of burning.

The powder grains of propelling charges designed for use in various guns are not all alike and cannot be used interchangeably. For example, the grains used in 16-inch gun propelling charges are not suitable for use in 20-mm gun ammunition.




Factors Affecting Projectile Initial Velocity (continued)

Two other factors which affect projectile I. V. in guns using bag ammunition are density and manner of loading. Density of loading is a measure of the amount of propelling charge used to fire a round. The higher the density of loading, the more propellant used; in general the muzzle velocity increases with increased density of loading.

Manner of loading refers, as the term implies, to the manner in which the propelling charge is placed in the gun breech, and also to the force used to position (ram) the projectile in the gun prior to loading the propelling charge. If the manner of loading is not kept the same, it will result in variations in muzzle velocity with resultant loss of accuracy. These factors do not affect the I. V. of projectiles fired from guns using fixed or semi-fixed ammunition; the propelling charges for these guns are fixed and hence there is no variation in density or manner of loading.

The temperature of the propelling charge has a very definite effect on projectile I. V. A typical 5-inch gun may have a normal I. V. of 2500 fps (feet per second) at a powder temperature of 90 degrees Fahrenheit. A temperature drop of 60 degrees from normal may decrease the I. V. as much as 100 fps.

When the first round is fired from a gun after it has been cleaned and oiled, the oil film on the surface of the bore slows down the projectile which must force the oil ahead of it through the tube. This reduction in I. V. on the first round must be compensated for by proper adjustments to the gun setting (position of gun in elevation and train).

1st round.  oil film reduces I.V., darn that oil, it slowed me down. Succeding rounds.  2nd round. Clean bore-normal I.V., Ah! A clean bore-watch my speed.




The Causes and Effects of Erosion

The wearing away of the gun bore during firing is called erosion. Your first guess would probably be that erosion is caused by the friction produced by the rubbing of the rotating band against the bore as the projectile travels through the gun tube, but surprisingly this is not the case. Some of the causes of erosion are: (1) the surface of the bore becomes intensely heated during firing and the hot gases rushing across the hot metal scour the surface, as would steel wool; (2) the rapid heating and cooling of the bore during and between rounds tends to weaken the metal and small surface cracks may develop; (3) hot gases escaping around the rotating band of the projectile during firing act as high-velocity jets, causing additional scouring and cutting of the bore's surface.

Causes of erosion. Scouring action of gasses. High velocity gas jets.

Erosion of the gun bore occurs mainly at the origin of rifling-the point in the gun bore near the breech where rifling begins. In guns using bag or semi-fixed ammunition, the projectile may be rammed further into the gun which has the effect of reducing the density of loading, and hence the muzzle velocity is decreased.

Erosion also causes the tops of the lands of the rifled bore to wear away faster than the bottoms of the grooves, which increases the amount of gas which escapes past the rotating band. This further reduces the pressure behind the projectile and thus decreases muzzle velocity.

Exaggerated cross-section of normal and eroded gun bores.




The Causes and Effects of Erosion (continued)

You have seen that erosion of the gun bore is due primarily to the high temperature and velocity of the expanding gases during firing, and the alternate heating and cooling of the gun barrel. The longer the hot gases remain in the gun, the greater the erosion; the more time allowed to elapse between rounds, the more the barrel cools, increasing wear due to change in temperature. Thus, large guns with their longer barrels (which allow gases to remain in gun longer) suffer more from erosion on each round fired than do small guns.

Small guns suffer less from erosion on each round fired than big guns.

Even a slight amount of erosion reduces the muzzle velocity, range and accuracy of a gun. The gun eventually becomes unserviceable when erosion reduces its effectiveness to a point where the projectiles fired no longer hit the target. Before a gun becomes inaccurate due to erosion, the barrel must be replaced or relined (in guns equipped with removable barrel liners). The life of a big gun is less in terms of rounds fired than that of a machine gun.

A record is kept of the number of rounds fired by each gun so that proper information on bore erosion can be supplied for fire control purposes, and to enable the gunnery officer to know when the barrel needs replacing.




The Use of Arbitrary Ballistics

When the target has been located, its range and bearing determined, interior and exterior ballistics corrections made, and all other details of the fire control problem have been worked out, the guns are fired. If all calculations and corrections have been properly made, a hit should be scored. Occasionally, however, it may be noted that the projectiles from one particular gun consistently fall some yards short or beyond the target. Despite all of the measurements or adjustments made, no cause can be determined for the consistent error of the gun. In such a case, a correction is applied to that gun's settings to compensate for the consistent error, and this correction is called an "arbitrary ballistic."

Arbitrary ballastics correct for consistent errors.  With and without ballistic corrrection showing hit and miss.

The need for and the amount of arbitrary ballistic correction can be determined only by observing the fall of a great many rounds. The fact that one or two rounds fall short or beyond is not justification for application of an arbitrary ballistic. A careful study of all the factors of the fire control problem must be made before it is decided to use an arbitrary ballistic.




Interior and Exterior Ballistics

On the preceding sheets you learned about guns and their construction, gun mounts, the types and characteristics of explosives. You also learned about various types of ammunition and projectiles. Finally you studied the meaning of interior ballistics and learned about the factors which affect a projectile during its travel through the gun bore. Now you should have a pretty good idea of what is required to start a projectile on its trip to the target. You have the gun properly mounted and loaded with a complete round of ammunition. The propelling charge is ignited, the projectile travels through the barrel under the influence of the interior ballistics factors and emerges from the gun muzzle traveling at a fixed speed and in a certain direction. What then?

Exterior ballistics? Interior ballistics.  I'm out of the gun according to plan-now what?

At the instant the projectile leaves the gun its initial velocity and direction are fixed and the influence of the interior ballistics of the weapon is ended. The course which the projectile follows after leaving the gun muzzle is determined by exterior ballistics. Thus interior and exterior ballistics are related by projectile initial speed and direction.

In the next section you will study exterior ballistics-an analysis of the factors which affect a projectile during its flight from gun muzzle to target.

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