Lines, Planes, Area, and Angle of Gunnery in Forensic Ballistics
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The axis of the piece is a straight line passing down the centre of the bore.
The axis of the trunnions is a straight line passing through the centre of the trunnions, at right angles to the axis of the piece.
The line of sight is a straight line, passing through the sights of the piece and the point aimed at.
The line of fire is a straight line from the muzzle of the piece to the point aimed at. This term would be used instead of the preceding, if the ordinary sights cannot be used.
The line of departure is the direction in which the projectile is moving on leaving the piece; or in other words, a tangent to the trajectory at the muzzle.
The trajectory is the curve described by the projectile in passing from the muzzle of the piece to the first point of impact.
The calibre is the diameter of the bore measured in inches. In rifled pieces, it is measured across the lands.
Clearance is the linear distance between the lower part of the body of the projectile and the bore of the gun, or it may be defined as the difference between the height of the stud and the depth of the groove.
Range is the distance from the muzzle of the piece to the second intersection of the trajectory with the line of sight, but on the practice ground the distance of the target from the gun (in yards) is called the range.
Drift is the constant deflection of the projectile from the plane of departure due to the rotation imparted by the rifling of the piece. It is sometimes termed derivation.
Lateral deviation is the perpendicular distance of the point of impact of the projectile right or left of the plane of sight.
The planes of sight and departure are vertical planes passing through the lines of sight and departure respectively.
Windage is the difference between the sectional area of the gun through its grooves, and of the body of the projectile through its studs. Windage is sometimes expressed in linear units; it is then the difference between the diameter of the projectile and the calibre of the gun, irrespective of studs or grooves.
The angle of sight is the angle which the line of sight makes with the horizontal plane (S, Fig. 1).
The quadrant angle is the angle which the axis of the piece, when laid, makes with the horizontal plane. It is termed quadrant elevation or depression, according as the piece is laid above or below the horizontal plane (Q, Fig. 1); the term depressed fire means that a piece is fired at a quadrant angle of depression.
The angle of departure is the angle which the line of departure makes with a horizontal plane (D, Fig. 1).
The angle of elevation is the angle between the axis of the piece and the line of sight (E, Fig. 1).
The angle of projection is the angle between the line of departure and the line of sight (P, Fig. 1).
Jump is the angle between the line of departure and the axis of the piece before firing (J, Fig. 1).
The angle of descent is the angle which a tangent to the trajectory at the first point of impact makes with the horizontal plane (B, Fig. 1).
