The gunsmith's manual

by J. Parish Stelle; Wm. B. Harrison

Figure 30.

Old Method of Straightening Barrels.—The old-time method of straightening a gun-barrel was by means of a fine thread of black silk or a hair, which was passed through the bore of the barrel. This line was drawn tight by being stretched from two ends of a rod of wood or spring steel, the elasticity of which kept it taut, and the workman looking through turned the barrel round so as to bring the thread of hair into coincidence successively with every portion of the inner surface. If there existed any concavity in any part of this surface, the thread would show it by the distance which would there appear between the thread itself and its reflection in the metal.

The New Method.—There is another process of straightening barrels which was explained by a writer in a scientific paper a few years ago, which is termed “straightening by the shade,” and by this method barrels can be straightened with a greater degree of precision than by any other known process. The principle is something like this: If we examine a plane mirror for the purpose of ascertaining whether its reflecting surface is a true plane, we cause objects to be reflected from it to the eye at small angles of incidence. If under these circumstances every part of the mirror gives an image true to nature, he pronounces it perfect; for the slightest deviation from a true plane would cause a manifest distortion of the image. In the process of straightening barrels by the shade, crooks in the bore are detected upon the same principle. The internal surface of the barrel is a mirror, and whatever objects are reflected to the eye from any portion of it that lies beyond a certain distance, will be reflected under very small angles of incidence. As the interior surface of the barrel is not a plane mirror, the reflected image will not be true to nature. If the bore be straight, the image will have a normal distortion, which is due to the transverse or cylindrical curvature of the mirror; while if there be longitudinal flexures or crooks, there will be an abnormal distortion of the image, which will reveal the defect.

Figure 31.

When the eye looks into a gun barrel the interior surface appears to be spread out into a plane circular disk, as far from the eye as the other end of the barrel. Through the centre of this disk is a circular orifice, and surrounding this at equal distances from it and from each other, respectively, are several well-defined concentric circles, dividing the disk into as many bright concentric rings, each of an apparent breadth, precisely equal to the diameter of the central orifice which is the other end of the bore as seen by direct vision. The several concentric circles are so many images of the end of the bore reflected to the eye from different points along its length. The first of these circles, or that nearest the central orifice, is an image formed by light once reflected. The second, third, fourth, etc., respectively, are images formed by light reflected two, three, four, times, etc. In order to see how these images are formed, and to find their respective points of location in the bore, consider that a ray of light from each point in the end of the calibre, as shown at a, [Fig. 31], may pass to same point b, on the other side of the bore, and be thence reflected to the eye, thus forming at b, an image at the end of the bore, of one reflection. Another ray from the same point may pursue the route a, c, d, e, forming an image at d of two reflections. Another ray may take the route, a, f, g, h, e, forming an image at h by three reflections, and so on for the other images since in the formation of each of these images, respectively, the angles of incidence and reflection are equal, it follows that the focus, or point of place of the image b, formed by one reflection of light, is at one-third of the distance from the eye to the further end of the bore; that formed by two reflections d, is one-fifth; that by three reflections is one-seventh, and the succeeding ones, one-ninth, one-eleventh, etc., of the same distance.

Therefore, it will be observed that all these images are located within the third part of the length of the bore nearest to the eye. Consequently there are two-thirds of the entire length of the bore in which none of these images appear. It is to this part of the bore, only that the workman directs his attention, for it is here only that he can cause the “shade” to appear which discloses the crooks in the bore if any exist. When this part is straightened, he inverts the barrel and works from the other end.

Figure 32.

The practical application of the process is made in this manner: the workman has a rest, generally consisting of an upright strip of board of convenient height with a V cut in its upper end for convenience in resting the barrel in case of rotating it. Across a window opposite, at almost any distance, say about ten or twelve feet, is nailed horizontally, a strip of board like a common lath, as the horizontal bar of the window sashes where they come together at the middle of the window (provided there be upper and lower sashes) will answer nearly as good a purpose. Now place one end of the barrel in the V of the rest, look into the bore, directing the eye to the lower side of it and to the point just beyond the image b; gradually depress the end held in the hand, bringing the direction nearer and nearer to the horizontal strip, or the sash bar as the case may be, and a dark shade is soon seen as shown at m, [Fig. 32]. This is the reflected image of the horizontal strip, or sash bar, the curved part of the outline being the image of the straight-edge. Depress the end more and more and the shade lengthens to n, o, p, etc. If the bore be perfectly straight, the shade will always maintain a true and symmetrical parabolic form, growing more and more pointed at its apex, until it reaches the further end of the bore. But if there be even the slightest flexure or crook in the bore the parabolic figure of the shade will be distorted. If a distortion be discovered, the barrel is slowly revolved about its axis as it is retained in the rest; at the same time slightly elevating and depressing the end held in the hands, until the shade assumes a form in which the two sides near the apex are equally drawn in toward each other as shown at q. If the crook be considerable the two sides may be drawn quite together, cutting off a portion of the shade of the apex as at r. This tells that there is a crook at q, and also tells that the bore is concave downward at that point. It will require some experience to tell how far that point is from the eye, but when that is learned, the fore-finger placed upon this point on the under side of the barrel tells where the blow must be given to straighten it as it ought to be.