Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/297

MILITARY EIFLES.] GUN MAKING 283 to time. It will be sufficient if the principle of these operations are described, without entering minutely into details. The rifle is composed of three parts : the barrel, carrying the sights ; the stock, into which fits the cleaning rod ; the body, containing the lock and breech action. The soldier is also provided with a bayonet to fit on the muzzle. Fia. 14. Martini breech action (closed). Jarrel, The barrel is made of mild steel manufactured on the Siemens principle. Cylinders of this material, 16 inches long and 1^- in. thick, are supplied by contract, and tests are made of a proportion from time to time. These cylinders are first rolled in the factory to a long cone, having slightly greater dimensions than the exterior of the FIG. 15. Martini breech action (open). finished barrel. The cylinder is heated in the furnace and passed straight through a series of eight pairs of roll ; each pair is grooved to receive it, the grooves becoming narrower and shallower as each successive pair is reached. The pairs are alternately vertical and horizontal ; when they are in full work they can roll about 250 cylinders per hour. FIG. 16. Henry barrel. The cone is next placed between friction rollers ; these are set at a slight angle, so that in revolving they pass the cone along slowly. It emerges from their clutch polished with the compression, arid showing their action by a close spiral traced on the surface. We have now a solid piece of mild steel, slightly thicker than the barrel, fairly straight and thoroughly well-consolidated. 2 FIG. 17. Henry rifling (true size and magnified). The next step is to obtain true bearings for boring the interior and turning the exterior. The cone is placed in a clamp milling machine, and the ends milled down. This operation requires some judgment ; the milling-down of the ends must be so performed that the centres shall be ap proximately true centres ; that is, the amount of metal to be sub sequently turned down off the ex terior of the bar rel shall be about equal all round. The ends, when milled down, serve for bearings by which the cone is next held horizon tally, and drilled in a double-acting machine for in.; these act as centres when the barrel is placed vertically and drilled through from each end. Three tools are used, the last being of slightly greater diameter than the others. A shoulder is left in the centre to avoid the inconvenience of the tools from opposite ends not exactly meeting in the middle, as might be the case with the long slender drills necessarily employed. This shoulder is removed by a square tapered tool or " bit." We have now a cone slightly larger than the barrel, with a hole through it -433 in. in diameter. This hole is then bored out to 444 inch. Having now got a true inside, the next step is to obtain a true outside from it. The cone is placed vertically in a machine, the lower end fitting accurately on a pin, the upper end passing loosely through a hollow chuck, which revolves round a centre in truth with the centre of the pin and the axis of the bore of the cone ; the exterior of the cone, being slightly irregular, will be out of truth; sulphur therefore is melted and poured into the hollow chuck, filling up the space between the interior of the chuck and the exterior of the cone, and hardening on cooling. The chuck then holds the cone by the outside in truth, and serves as a bearing. The cone is then placed in a lathe, and two or three short cuts taken off the outside as bearings for the rough turning. This is called "spotting." It is now rough and finish turned, the outside becoming concentric with the inside ; and the cone becomes a barrel. It is next tested for truth of interior. Two tests are Testing employed, the first being that by shading. The barrel is barrels placed in a rest, so arranged that the eye looking through the tube sees the straight line of the top of a window cutting horizontally across the aper ture a little above the cen tre. If the barrel is accu rately straight and brightly polished, the shadow thrown by the dark window-frame down the tube appears as in fig. 18. The interior circle shows the window as seen through the barrel, with the dark frame cutting across ; FI G. 18. Testing by shading, the next circle shows the shadow extending in a cone from A to B and C. This shadow is quite dark, and AB, AC are quite straight. BC is a portion of the arc cutting off

the shadow sharply at a point half-way down the barrel.