The object of this rotation is to increase the range of a projectile, by causing it to move through the air in the direction of its least resistance, and to correct the cause of deviation by distributing it uniformly around the line of flight.
Various plans have been tried to secure the safest and surest means of causing the projectile to follow the spiral grooves as it passes along the bore of a rifled piece. Those projectiles, which promise to be the most successful for heavy guns, may be ranged under two heads, viz:
1st. Those which have flanges or projections on them to fit into the grooves of the gun in loading.
The flanges are made of softer metal than the body of the projectile.
2d. Those which are constructed on an expanding principle.
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The body is generally made of cast-iron; and the expanding portion is a band or cup of some softer metal, as pewter, copper, or wrought iron, which enters the bore of the piece freely when it is loaded, but which is forced into the grooves by the discharge.
The grooves are of different forms, determined by the angle made by the tangent line at any point with the corresponding element of the bore. If the angles be equal at all points, the groove is said to be uniform. If they increase from the breech to the muzzle, the grooves are called increasing; if the reverse, decreasing grooves. The practical method of cutting grooves consists in moving a rod armed with a cutter, back and forth in the bore, and at the same time revolving it around its axis. If the velocities of translation and rotation be both uniform, the grooves will have a uniform twist; if one of the velocities be variable, the grooves will be either increasing or decreasing, depending on the relative velocities in the two directions.
Twist is the term employed by gunmakers to express the inclination of a groove at any point, and is measured by the tangent of the angle which the groove makes with the axis of the bore; and this is always equal to the circumference of the bore divided by the length of a single revolution of the spiral measured in the direction of the axis.
The most suitable inclination of grooves for a rifle cannon has not yet been determined experimentally; and consequently a wide diversity of twists is employed by different experimenters.