Causes which affect Initial Velocity

—The principal causes which affect initial velocity are variations in the weights of the powder and projectile, the manner of loading, the temperature of the piece, and the balloting of the projectile along the bore. Rotation. The principal cause of the deviation of a projectile is its rotation combined with the resistance of the air. By balloting. If the projectile be spherical and homogeneous, balloting of the ball in the bore, arising from the windage. In this case the axis of rotation is horizontal, and pa es through the centre of the ball; the direction of rotation depends on the side of the projectile which strikes the surface of the bore last. The velocity of rotation from this cause depends on the windage, or depth of the indentations in the bore, the charge being the same. By eccentricity. If, from the structure of the ball, or from some defect of manufacture, the centre of gravity does not coincide with the centre of figure, rotation generally takes place around the centre of gravity. This arises from the fact that the resultant of the charge acts at the centre of figure, while inertia, or resistance to motion, acts at the centre of gravity. For the same charge the velocity of rotation pa es through the centre of gravity, and is perpendicular to a plane containing the resultant of the charge and the centres of figure and gravity. For the same charge, the velocity of rotation is proportional to the lever arm, or the perpendicular, let fall from the centre of gravity to the resultant of the charge. Knowing the position of the centre of gravity of the ball in the bore, it is easy to foretell the direction and velocity of rotation. In general terms the front surface of the projectile moves toward the side of the bore on which the centre of gravity is situated, and the velocity of rotation is greatest when the line joining the centres of gravity and figure is perpendicular to the axis of the bore.