killing is done by pushing in from the muzzle a bar, carrying a planer tool to cut the grooves, and so rotated ]n• rack-and-pinion and a forming bar, or by a guide-pin and groove in the bar, as to make the groove of the prescribed twist. The curve of the rifling, to reduce resistance at first, increases from one turn in 50 calibres, at the commencement, to one turn in 25 calibres, at the muzzle (the United States standard), and is, when developed on a plane surface, a semi cubic parabola. The forming bar referred to, being set to that curve as calculated for the particular gun. guides a straight bar, perpen dicular to the rifling bar, in such a manner that by means of its rack and the rifling bar's pinion the proper rotation is imparted to the toot.
The powder chamber is in rear of the rifled part of the gun, and is of slightly larger diameter to hold more powder in a given length, and to allow for a narrowing cone at its front end to stop the projectile in the right position and bring it to a central position. Forward of this cone is the rifling. but, to prevent excessive pressures due to instantaneously overcoming the whole resistance of the rotating band to taking the rifling, it is customary to ream off about half of the depth of the lands (or ridges of metal between the rifling grooves) at this point, and this reaming is done in the form of a cone several feet long, so that the increase in height of the lands shall be gradual throughout this distance. The breech recess is the part in rear
of the tube which is threaded and slotted for the breech block.
Each gull is required in the United States (and all nations have similar requirements) to be fired at least five rounds before it is issued for use in fortifications. Before the adoption of a new type of gun, one gun of this type must have withstood 500 rounds.
In the manufacture of wire-wound guns the tube must be machined as for the built-up gun, and is wound by rotating it in a lathe white the wire is fed from spools through a machine, giv ing the wire the tension calculated therefor. Whatever jacket and hoops are to be used in the design are shrunk on exactly as in built-up guns.