But 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 required range, and is found to gain in accuracy by increasing this rotatory 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 out without rotation. The English gunsmiths have avoided the dilemma by giving the requisite pitch and making the grooves very deep, and even by having wings 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 have 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.
English makers have experimented somewhat on the rifling of barrels, but with no results which compare with those shown by the improved Kentucky. English hunting-rifles, and all military rifles, are made with complete disregard of the law of relation between the weights of ball and barrel. The former seems to be determined by dividing the weight of ammunition a soldier may carry in his cartridge-box by the number of charges he is required to have, and then the gun is made as light as will stand the test of firing,—blunders all the way through; for we never want a rifle-ball to range much farther than it is possible to hit a single man with it; and a missile of the proper shape from a barrel of sixty gauge will kill a man at a mile's distance, if it strike a vital part. The consequence is, that the rifles are so light in proportion to their load that the recoil seriously diminishes the force of the ball, and entirely prevents accuracy of aim; and at the same time their elastic metal springs so much under the pressure of the gas generated by the explosion of the powder that anything like exactitude becomes impossible.[1][Footnote 1: Experiments have shown, that, with a barrel about the thickness of that of our "regulation rifles," the spring will throw a ball nearly two feet from the aim in a range of six hundred yards, if the barrel be firmly held in a machine.] This the English gunsmiths do not seem to have learned, since their best authorities recommend a gun of sixty-four gauge to have a barrel of four pounds weight, and that is considered heavy,—while ours, of sixty gauge, would weigh at least twice that. To get the best possible shooting, we find not only weight of barrel requisite, but a thickness of the metal nearly or quite equal to the diameter of the bore.
Mr. Whitworth, of Manchester, revived the old polygonal bore, and, by a far more perfect boring of barrel than was ever before attained in England, has succeeded in doing some very accurate shooting; but the pitch of his grooves requisite to give sufficient rotation to his polygonal missile to enable it to rotate to the end of its flight is so great, that the friction and recoil are enormous, and the liability to burst very great, Mr. Whitworth's missile is a twisted prism, corresponding to the bore, of two and a half diameters, with a cone at the front of one half the diameter. Such a gun, in a firing-machine, with powder enough to overcome all the friction, and heavy enough to counteract torsion and springing, would give very great accuracy, if perfectly made, or as well made as American rifles generally; but no maker in England, not even Mr. Whitworth, has attained that point yet; and even so made, they would never be available as service—or hunting-guns.
The Lancaster rifle avoids grooves (nominally) altogether, and substitutes an elliptical bore, twisted to Mr. Whitworth's pitch (twenty inches). General Jacob says, very justly, of this gun: "The mode of rifling is the very worst possible. It is only the two-grooved rifle in disguise. Let the shoulders of the grooves of a two-grooved rifle be removed, and you have the Lancaster rifle. But by the removal of these shoulders, the friction, if the twist be considerable, becomes enormous." To compare this twist with the rifled bore, one has only to take a lead tube, made slightly elliptical in its cross-section, and, fitting a plug to its ellipse, turn the plug round, and he will see that the result is to enlarge the whole bore to the longest diameter of the ellipse, which, if it were a gun-barrel, unelastic, would be equivalent to bursting it. But this is exactly the action which the ball has on the barrel, so that, to use General Jacob's words, "the heat developed by the friction must be very great, and the tendency of the gun to burst also very great." Lieutenant Busk—who seems, if we may judge from the internal evidence of his book, to know little or nothing of good rifles or rifle-practice, and to have no greater qualification for writing the book than the reading of what has been written on the subject and an acquaintance of great extent with gunsmiths—remarks, in reply to the veteran of English riflemen: "Having given the matter the very closest attention, I am enabled confidently to state that the whole of this supposition [quoted above] is founded in error…. So far from the friction being enormous, it is less than that generated in any other kind of rifle. It is also utterly impossible for the bullet to act destructively on the barrel in the way suggested." Such cool assurance, in an unsupported contradiction of experience and the dictates of the simplest mechanical common-sense, would seem to promise little real value in the book, and promises no less than it really has.
The same objection which lies against the Lancaster rifle (?) applies to the Whitworth in a less degree. If the reader, having tried the lead-pipe experiment above, will next hammer the tube hexagonal and try the plug again, he will find the same result; but if he will try it with a round bore grooved, and with a plug fitting the grooves, he will see that the pressure is against the wall of the groove, and acts at right angles to the radius of the bore, having only a tendency to twist the barrel in order to straighten the grooves,—a tendency which the barrel meets in the direction of its greatest stability. We may see, then, that, in theory at least, 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 (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. The figure
[Illustration: ]
represents, on one side of the dotted line, three grooves, 1, 1, 1, cut in this way, exaggerated to show more clearly their character. In the Kentucky rifle this law is followed, except that, for convenience in cutting, the grooves are made of the same width at the bottom and top, as shown at 2, 2, 2, which is, for grooves of the depth of which they are made, practically the same, as the dotted circle will show. Our gun-makers use from six to ten grooves.
To sum up our conditions,—the model rifle will conform to the following description:—Its weight will be from ten to twelve pounds; the length of barrel not less than thirty inches,[1] and of calibre from ninety to sixty gauge; six to ten freed grooves, about .005 inch deep, angular at bottom and top, with the lands of the same width as the grooves; twist increasing from six feet to three feet; barrel, of cast steel,[2] fitted to the stock with a patent breech, with back action set lock, and open or hunting and globe and peek sights. Mr. Chapman, whose book is the most interesting and intelligent, by far, of all hitherto published, recommends a straighter stock than those generally used by American hunters. Here we differ;—the Swiss stock, crooking, on an average, two inches more than ours, is preferable for quick shooting, though in a light rifle much crook in the stock will throw the muzzle up by the recoil. With such a gun,—the best for hunting that the ingenuity and skill of man have ever yet contrived and made,—one may depend on his shot, if he have skill, as he cannot on the Minié, Enfield, or Lancaster; and whether he be in the field against a foe, or in the forest against the deer, he holds the life of man or deer in his power at the range of rifle-sighting.
[Footnote 1: There is much difference of opinion amongst gun-makers as to the length of barrel most desirable. We believe in a long barrel, for the following reasons: 1st, a longer distance between sights is given, and the back sight can be put farther from the eye, so that finer sighting is possible; 2d, a long barrel is steadier in off-hand shooting; 3d, it permits a slower powder to be used, so that the ball starts more slowly and yet allows the full strength of the powder to be used before it leaves the barrel, getting a high initial velocity with little recoil, and without "upsetting" the ball, as we shall explain farther on. The experiments of the United States government show that the increasing of the length of the barrel from thirty-three to forty inches (we speak from memory as to numbers) increased the initial velocity fifty feet per second; but this will, in long ranges, be no advantage, except with such a shape of missile as will maintain a high speed.]