Uniform Groove.—The comparative advantages of uniform and variable grooves depend on the means used to connect them with the projectiles. If the bearing of the projectile in the grooves be long, and the metal of which it is made be unyielding, it will be unsafe, if not impracticable, to employ variable grooves, and if the metal be partially yielding, a portion of the force of the charge will be expended in changing the form of that part of the projectile which projects into the grooves, as it moves along the bore.
Variable Groove.—The variable groove may be used to advantage when the portion of the projectile in the grooves is so short that its form will undergo but slight alteration; the variable groove diminishes the friction of the projectile when it is first set in motion, and thereby relieves the breech of the piece of a portion of the enormous strain which is thrown upon it. If the twist be too rapid towards the muzzle there will be danger of bursting the piece in the chase.
Width of Groove.—The width of a groove depends on the diameter of the bore and the peculiar manner in which the groove receives and holds the projectile. Wide and shallow grooves are more easily filled by the expanding portion of the projectile than those which are narrow and deep; and the same holds true of circular-shaped grooves when compared to those of angular form.
Number of Grooves.—An increase in the number of grooves increases the firmness with which a projectile is held, by adding to the number of points which bear upon it. A large number of grooves, however, increase the difficulties of loading. For expanding projectiles an odd number of grooves is generally employed, for as this places a groove opposite to a land, less expansion will be required to fill them.
Inclination of Grooves.—Experience has shown that, as the velocity of rotation depends upon the form and initial velocity of the projectile, the causes which retard it, and the time of flight, there is a particular inclination of grooves which is best suited to each caliber, form of projectile, charge of powder, and angle of fire. The farther the centre of gravity of a projectile is in rear of the centre of figure, or resistance of the air, the greater must be the inclination of the grooves to resist deviation. It therefore follows that a conical projectile of the same length and diameter, requires a greater inclination of grooves than a cylindrical projectile, and the same will hold true for other forms as they approach one or the other of these extreme cases. The friction of the projectile as it passes along the grooves, increases with their inclination; its effect will be to diminish the range and increase the strain on the piece, and the inclination may be carried so far as to break the projectile, or rupture the piece.
Centring.—In consequence of the windage necessary in all muzzle-loading guns, the axis of the projectile does not always coincide with that of the bore in firing. This leads to inaccuracy of fire. A projectile is said to be centred when the grooves of the rifling are so constructed as to bring the axis of the projectile on a line with that of the bore when the piece is fired. There are several ways of accomplishing this, among them Armstrong’s method called “Shunt rifling.” See [Armstrong Gun] under [Ordnance, Construction of], the [Krupp], and others.
Riga. An important seaport of Russia, capital of Livonia, on the Düna, 312 miles southwest from St. Petersburg. Riga was founded in the beginning of the 13th century. The Teutonic Knights possessed it in the 16th century. In 1621 it was taken by Gustavus Adolphus, and in 1710, after a vigorous resistance, by Peter the Great. On the latter occasion more than half of the town was destroyed. In 1812 a French force was repulsed from the town.
Rigodon (Fr.). Formerly a beat of drum while men who were shelled (a French punishment, the severest next to death) were paraded up and down the ranks previous to their being sent to their destination.
Rimbases. In gunnery, are short cylinders uniting the trunnions with the body of the gun. The ends of the rimbases, or the shoulders of the trunnions, are planes perpendicular to the axis of the trunnions. Rimbases are for the purpose of strengthening the trunnions at their junction with the piece, and by forming shoulders, to prevent the piece from moving sideways in the trunnion-beds.
Rimer. A palisade in fortification.