CHAPTER IV.

GUNNERY.

=96.= _General instructions._

1. The subjects dealt with in this chapter are only divided into sections for purposes of reference. The contents of many of the sections are so closely connected that portions of several of them must often be included in one lecture. In addition it must be realized that it is impossible to deal fully within the limits of a manual with the various subjects mentioned. All that is attempted is to give a brief outline of the principles of modern gunnery.

2. A description of the various patterns of ordnance used in the field, their stores and drill will be found in the respective handbooks; while for further information of a more technical nature reference should be made to the “Text Book of Gunnery,” the “Treatise on Ammunition,” and the “Treatise on Service Explosives.” Instructors should endeavour to use the simplest language; but, as a large number of more or less technical terms must be employed, they must take care that their meaning is clear.

=97.= _Gunnery terms._

_Angle of Departure._--The angle which the line of departure makes with the horizontal plane, in other words, the quadrant angle plus the jump. (_See_ 5, Plate II.)

_Angle of Descent._--The angle which the trajectory makes with the line of sight at the point of their second intersection.

_Angle of Incidence._--The angle which the trajectory makes with the normal to the surface struck.

_Angle of Elevation._--The angle which the line of sight makes with the axis of the gun. (_See_ 2, Plate II.)

_Angle of Sight._--The angle which the line of sight makes with the horizontal plane. (_See_ 3, Plate II.)

_Axis of the Gun._--A line passing down the centre of the bore. (_See_ A B, Plate I.)

_Axis of the Trunnions._--A line passing through the centre of the trunnions. (_See_ C D, Plate I.)

_Battery Angle._--The angle formed at the battery by imaginary lines drawn to the target and the observing station.

_Calibre._--The diameter of the bore in inches measured across the lands.

_Drift._--The constant deflection of the shell due to the rotation imparted by the rifling. (_See_ A, Plate III.)

_Direct Laying._--When the gun is laid by looking over or through the sights at the target.

_Indirect Laying._--When the gun is laid for direction on an aiming point, or on aiming posts, the angle of sight is adjusted by clinometer and the elevation by the range indicator or drum. (With the 5“ B.L. howitzer the range drum is set at the quadrant elevation.)

_Jump._--The angle between the line of departure and the axis of the piece before firing. It is due to the vertical movement of the gun on firing, and for any gun differs according to the mounting and the charge used. (_See_ 6, Plate II.)

_Lateral Deviation._--The distance of the point of impact of the projectile right or left of the line of fire.

_Line of Departure._--The direction of the shell on leaving the muzzle. (_See_ 4, Plate II.)

_Line of Fire._--A line joining the muzzle of the piece and the target.

_Line of Sight._--A straight line passing through the sights and the point aimed at. (_See_ E F, Plate III.)

_Muzzle Velocity_--The velocity in feet per second with which a shell leaves the muzzle.

_Point Blank._--A gun is laid point blank when the line of sight is parallel to its axis.

_Quadrant Angle._--The angle which the axis of the piece makes with the horizontal plane. It is termed quadrant elevation or depression according as the gun is laid above or below the horizontal plane. (_See_ 1, Plate II.) The angle of elevation and the quadrant angle are the same when the line of sight is horizontal.

_Range._--The distance to the second intersection of the trajectory with the line of sight.

_Ranging._--Ranging is the process of finding the elevation, fuze, and line.

_Remaining Velocity._--The velocity of a shell at any given point of its trajectory.

_Striking Velocity._--The velocity of a shell at the point of impact.

_Trajectory._--The curve described by the shell in its flight. (_See_ D F, Plate III.)

=98.= _Natures of artillery fire._

(_See_ Fig. 11.)

_High Angle Fire._--Fire from guns and howitzers at all angles of elevation exceeding 25 degrees. But in coast defence the term “High Angle Fire” is used in connection with guns of mountings specially designed for extreme angles of elevation laid by means of instruments specially provided.

_Enfilade Fire._--Fire which sweeps a line of troops or defences from a flank.

_Frontal Fire._--When the line of fire is perpendicular to the front of the target.

_Oblique Fire._--When the line of fire is inclined to the front of the target.

_Reverse Fire._--When the rear instead of the front of the target is fired at.

=99.= _Gunnery._

1. Gunnery is the science of directing a projectile so that it will strike a given object.

2. A gun serves two purposes. First to confine the gases of the charge so as to allow them to act upon the base of the shell; and second to give the shell the proper initial direction. In order to maintain the shell in its proper direction after it leaves the bore of the gun, some means of making it spin or rotate rapidly is necessary; since it is a well-known fact that any rapidly-rotating body tries to keep the same direction in which it was pointed when first made to spin. Rifling which consists in a number of grooves cut down the bore, leaving raised ribs called “_lands_” between them, is the means employed in modern guns, in combination with a soft copper band called a “_driving band_” secured to the shell near its base. The result of this combination is that when the gun is fired the shell is forced along the bore, but, the diameter of the driving band being bigger than that of the bore, the lands cut into the copper of which it is made and the shell is consequently compelled to follow their course and rotate. Without this spin an elongated shell would soon lose its velocity and accuracy of direction and become as unreliable in its flight as round shot were in the days of smooth bore guns.

3. By the use of elongated shell the following additional advantages besides accuracy of direction are obtained:--