Naval guns are fired by electric, percussion, combination (electric and percussion), or friction primers. In the United States service only the first three varieties are used, and all guns are being fitted to use the combination primers as rapidly as opportunity offers. The electric and percussion primers are small tubes of copper an inch and a half long with enlarged heads not unlike in shape the bottle-neck small-arm car tridges if we imagine the part of the cartridge which is of large diameter to be very short; the electric primer has a small wire projecting from it which connects to a firing circuit; the percus sion primer has only a cap in the head like a centre-fire cartridge. The combination primer is similar to a 32-calibre cartridge she41, but much longer; the cap in the head, if struck, will cause the primer to explode; and it is insulated from the shell of the primer so that if touched by the point of an electric contact piece the current will flow through the cap, then through a bridge of fine platinum wire, thence to the shell of the primer, to the gun, and to earth. As the current passes through the platinum bridge it heats it white hot, igniting the priming composition, and that in turn ignites the charge, as with other primers. Primers are fired in locks, of which there are three styles in use. The oldest, called Mark VI., is fitted for electric or percussion firing, but to change from one to the other it is necessary to remove the wedge or breech-block of the lock. Mark VIII. is fitted for use with combination primers; to change from one method of firing to the other, it is only necessary to connect or dis connect the electric firing wire. The only other lock in service uses the combination primer, and differs from Mark VIII. in automatically ejecting the primer after firing. It is used on quick-fire guns. In all cases, the lock screws on the rear end of the mushroom stem. Somewhat similar locks are used in foreign navies. To prevent premature firing of guns—for it is convenient to insert the primer while loading is going on—the electric circuit is not complete through the primer until the breech-block is closed and fully turned into place, so that the contacts placed in the proper position are pressed together; the pressure of the firing key will then complete the circuit. In quick-fire and rapid-fire guns (see RAPID-FIRE GUNS) , a detent holds the percus sion firing pin back so that it cannot strike the primer until the breech is closed and locked. The electric current for firing is obtained from elec tric batteries at the guns or a current of reduced voltage derived from the dynamo.
The rifling of United States naval guns is poly groove with a twist which increases from zero to one turn in twenty-five calibres; that is to say, the projectile starts along grooves which at first are parallel to the axis of the bore, but which almost immediately begin to incline to form a spiral on its inner surface, the inclination increasing to a point near the muzzle from which the spiral is uniform; the projectile is thus caused to rotate faster and faster, until, when it leaves the muzzle, it is spinning at a rate that will make it revolve once while moving a dis tance equal to twenty-five times its diameter.
The grooves are .05-inch in depth and several times as wide, but decrease in width as the muz zle is approached, and their number is usually four times the calibre expressed in inches (for a 6-inch gun, 4 X 6 = 24) ; but there are some exceptions.
In the design of naval guns the weights are kept down as much as possible, as every addi tional ton of weight carried involves three or four tons extra displacement in the ship. In guns for forts and fixed defenses on shore, small sav ings in weight are unimportant. The largest guns now being made for ships of any navy are of 12-inch calibre, though many very new 13-inch are carried on recently built ships of the United States Navy and many 13.5-inch guns are in service in the British and French navies. The heaviest guns ever put on board ship were the 16.25-inch, 110-ton guns of the British battle ships Victoria, Sanspareil, and Ben bow. The first-named was lost in collision, so that the Sansparcil and Banbou, are the only vessels now carrying them. Aside from their unwieldy size, hich made them undesirable, these guns were faulty in design and were poorly constructed, so that they never gave satisfaction. Guns of 100 tons weight were mounted in Italian battleships many years ago, but some of these have been removed and replaced by others of less weight and smaller calibre, and the others are likely to be if the vessels are not condemned as not worth re-arming. At present, the general opinion in all navies is that the 12-inch gun is the largest neces-. nary or desirable to mount on board ship.
Wire-wound guns are used in the British ser vice, and some few guns of this type have been built for Japan, but the naval authorities of other countries do not favor them. Ample strength is usually secured by forged steel tubes, jacket, and hoops to stand the pressures de siiable with modern powders, but if transverse trunnions and resting on little wooden wheels called the trucks. The sides of the carriages were held together by framing called transoms, the rear one supporting the breech and elevating device. The latter consisted at first of a large wooden wedge, but was replaced by an iron ele vating screw. (See illustrations under ORD NANCE.) The recoil was checked by a heavy rope called the breeching, passing around the breech of the gun and secured at the ends to the ship's strength alone were to be considered there would be no objection to additional safety secured by wire-winding. The great defect of wire-wound guns is their lack of longitudinal stiffness, the wire not only failing to add any strength in this direction, but interfering with the stiffening due to the hoops. Should the time come when greater pressures than those now used are found desir able, then wire-winding may be resorted to, hut until that time arrives it is not likely to become general. Higher pressure in guns en tails an increase in temperature in the chamber and consequent additional erosion of the bore, and also adds to the difficulty of checking the escape of gas to the front around the projectile and to the rear around the breech gas-check.