Importance of Rifling.—In a rifle the grooving is of the utmost importance; for velocity without accuracy is useless. To determine the best kind of groove has been, accordingly, the object of the most laborious investigations. The ball requires an initial rotary motion sufficient to keep it “spinning” up to its range, and is found to gain in accuracy by increasing this rotary speed; but if the pitch of the grooves be too great, the ball will refuse to follow them; but being driven across them, “strips”—that is, the lead in the grooves is torn off, and the ball goes on without rotation. The English gun-smiths avoided the dilemma by giving the requisite pitch and making the grooves very deep, and even by having wings or lugs cast on the ball to keep it in the grooves—expedients which increase the friction in the barrel and the resistance of the air enormously.
The American gun-makers solved the problem by adopting the “gaining twist,” in which the grooves start from the breech nearly parallel to the axis of the barrel, and gradually increase the spiral, until, at the muzzle, it has the pitch of one revolution in three to four; the pitch being greater as the bore is less. This gives, as a result, safety from stripping, and a rapid revolution at the exit, with comparatively little friction and shallow groove-marks on the ball, accomplishing what is demanded of a rifled barrel, to a degree that no other combination of groove and form of missile ever has. There is no way of rifling so secure as that in which the walls of the grooves are parts of radii of the bore. They should be numerous, that the hold of the lands, or the projection left between the grooves, may divide the friction and resistance as much as possible, and so permit the grooves to be as shallow as may be. Fig. 41 represents grooves cut in this way, but exaggerated to show more clearly their character. In the Kentucky rifle this law is followed, except that for convenience in rifling, the grooves are made of the same width at the bottom and top, as shown in Fig. 42, which is, for the grooves of the depth of which they are generally made, practically the same, the depth in the cut being two or three times that generally used.
Figure 41.
U. S. Rifling Machines.—The rifling machines in use by the U. S. Government at the Springfield Armory for cutting their grooved rifles may thus be described: The barrel is placed in a horizontal position in an iron frame, and held there very firmly. The grooves are made by three short steel cutters placed within three mortices, made to receive them, near the end of a steel tube which is moved through the bore of the barrel by slow rotary and progressive motion. The cutters are narrow pieces of steel having upon one side three angular shaped teeth about one-sixteenth of an inch in height, and of the width of the groove, ground to a very sharp edge at the top. It is these which produce the rifling. The three cutters, when inserted in the tube, form upon their inner surface a small opening which decreases toward the inner end. Into this is inserted a tapered steel rod, and is so controlled by a connecting cog-wheel that this rod is pushed, at every revolution, a little further into the tapered opening formed by the inner edges of the three cutters. The effect of this is to increase the pressure of the cutters upon the inner surface of the barrel, and thus gradually, at each stroke of the machine, deepen the cuts as produced by the rifling. The rod makes about twelve revolutions in a minute and it occupies about thirty minutes to rifle a barrel.
Figure 42.
Old-Fashioned Rifling Machine.—But the gun-maker who builds rifles to order, and perhaps then but a single one at a time, uses quite a different apparatus for rifling, although the principle involved is the very same. Many of the old gunsmiths made their own rifling machines. The simplest form was a common joist, two inches thick and six inches wide. The length about twice that of any barrel to be rifled. At one end, on the narrow side, was fixed in two bearings, one at each end so as to turn freely, an old rifle barrel. At the other end of the timber, in a line with the barrel was fixed two standards in which to firmly fasten the barrel to be rifled. At the end of the old rifle barrel, and on the end nearest to the end of the timber was fixed a circular plate of iron, like a wheel which was made with divisions on its circumference, and had a catch which was fastened to the wood, and when the end of this catch engaged one of the divisions it would firmly hold the barrel in place. When this plate was turned the barrel also turned. Inside of this barrel was placed a rod of iron, around which was cast some soft metal, as babbit metal or old type metal, or even lead. This was done by putting the rod in the barrel and then pouring in the metal when melted. A handle, similar to an auger handle was fixed transversely to one end, but in such a manner as to turn around freely on the rod. As the rod was pushed back and forth the soft metal followed the rifling grooves, and this caused a turn, first in one direction and then in another. By the rod being loose on the handle the hands were held in the same position.
The rifling rod was attached to the opposite end of the rod in the old barrel and carried a cutter let into a narrow groove made in the extreme end of the rifling rod. Very often these rods were made of wood like a straight ramrod. It is evident if a barrel be placed in the clamps and the rifling rod bearing a suitable cutter be entered in the bore of the barrel and the rod thrust forward by pushing it with the handle, that a faint spiral like cut will be the result. When the cutter had done its work, or done all that it would cut, the plate was turned one division, rotating the old barrel with the rifling rod just that amount, the barrel to be rifled, of course, not being turned at all. Another cutting was done like the first one, then another, and so on until the complete circle of the divisions had been made, and a certain number of faint rifles made in the fixed barrel. The cutter was then removed from its slot, a slip of writing paper placed in the bottom of the slot and the cutter put in place and a repetition of the same operation gone over again, and then repeated until the rifling was of the desired depth. Oil was supplied to the centres while going back and forth in their work.
Gain Twist Rifling Machine.—What is called a gain twist was made with a slightly different apparatus. What is termed a “lead” was fixed so as to revolve in standards, and at the same time be capable of being thrust forward and pulled back to its starting place. This lead carried at one end the rifling rod. At the opposite end the handle for operating it, was fixed. The lead was several inches in diameter and the holes in the standards that supported it of like diameter. One make of lead had a groove cut spiralling in its surface and exactly the same as the rifling to be made. In one of the standards a stud was fixed that entered the groove and compelled it to turn according as the groove was made. The barrel to be rifled was fixed so as to turn as needed to make the different rifles or grooves. Another make of lead had a rib made of a strip of hard wood that was bent around the rod and was held in place by screws. This rib was the counterpart of the rifling and was received in a mortice cut for it in one of the standards, the management of the rifling rod being the same in all cases.
Re-rifling.—One method of re-rifling is to make a rod with a mortice in one end to receive a rifling cutter or “saw” as some term them, and fix at the other end a handle like an auger handle, but so fixed that the rod will turn around freely no matter how the handle may be held. This rod is inserted in the barrel to be re-rifled and the cutter forced through one of the rifles, which must be deep enough to force it to follow its direction when pushed forward and pulled back until it would cut no more, it would be placed in another rifle and so continued until the circuit of the rifling is made. A slip of paper is then put under the cutter and a repetition of the process made and continued until the rifles are cut as deep as desired.
If it be feared that the rifles, opposite to those where the cutter is at work, will be injured by its bearing upon them, a dove-tailed groove is made across the rod opposite where the cutter is placed, and in this is fitted a slip of wood that is cut to fit the curvature of the base of the barrel. If a bit of half-round file or a cutter be made to be inserted in place of the slip of wood, the lands can be finished at the same time that the grooves are being cut deeper.
When barrels are so worn that the rifles have not depth enough to hold the tool described for re-rifling, another process must be resorted to. Make a rod of hard wood about six or seven inches long and so as to slide easily through the barrel. In one end of this fix the cutter. Around the other end cast lead or other soft metal so as to fill the rifles. It is evident if this short rod be forced through the barrel by means of a longer one, that it must turn with the rifling, being so forced to do by the soft metal engaging the several rifles. The operation of working being the same as previously described.