Miscellaneous Papers on Mechanical Subjects/Rifled Fire-arms
RIFLED FIRE-ARMS.
Fire-arms are rifled to give rotation to the projectile round its axis of progression, in order to insure a regular and steady flight, the first thing therefore to be considered is what are the best mechanical means of effecting this object.
It would be foreign to our purpose to consider any of the various plans which have been proposed for furnishing the projectile itself with vanes, wings, grooves, or other configurations intended to give it rotation during its passage through the air. The only practical method hitherto adopted has been to make the barrel of a fire-arm of such a shape in its interior, that the projectile while being propelled from the breech to the muzzle may receive a rotatory combined with a forward motion.
The system of rifling by grooves is the one that has been generally employed, and many experiments with different numbers of grooves, some of varying depths being deeper at the breech, and with different turns, some increasing towards the muzzle, have been tried and thought advantageous at various times.
The Enfield rifle has three grooves, with a pitch of 6 feet 6 inches, so that the bullet receives half a turn round its axis while moving through the barrel, the length of which is 3 feet 3 inches. The bullet, as is well known, is cylindro-conoidal, it is wrapped in paper and made of such a diameter as to pass easily down the barrel. It requires very pure lead to allow of its being properly expanded or "upset," by the explosion, and is driven partly against the original portions of the bore, called the lands, and partly in the form of raised ribs is forced into the grooves, whose spiral shape gives the required rotation. See figure A.
In the system of rifling which I have adopted the interior of the barrel is hexagonal, and, instead of consisting partly of non-effective lands and partly of grooves, consists of effective rifling surfaces. The angular corners of the hexagon are always rounded, as shewn 
in section by figure C. Either cylindrical of hexagonal bullets may be used. See figures B, b. Supposing a bullet of a cylindrical shape to be fired, when it begins to expand it is driven into the recesses of the hexagon, as shown in figure C. It thus adapts itself to the curves of the spiral, and the inclined sides of the hexagon offering no direct resistance, expansion is easily effected.
It is most important to observe, that with all expanding bullets proper powder must be employed. In many cases this kind of bullet has failed, owing to the use of a slowly-igniting powder, which is desirable for a hard metal projectile, as it causes less strain upon the piece but is unsuitable with a soft metal expanding projectile, for which a quickly-igniting powder is absolutely requisite to insure a complete expansion, which will fill the bore. Unless this is done the gases rush past the bullet, between it and the barrel, the latter becomes foul, the bullet is distorted, and the shooting must be bad. With regard to fouling, it may be prevented by using a proper quantity of lubricating substance, of the right kind and adapted to the temperature of the weather, and a proper charge of powder of the right quality.
If the projectile used be made of the same hexagonal shape externally as the bore of the barrel internally, that is, with a mechanical fit, metals of all degrees of hardness, from lead, or lead and tin, up to hardened steel, may be employed, and slowly-igniting powder, like that of the service, may be used.
Having noticed the form of interior which provides the best rifling surfaces, the next thing to be considered is the turn, that is, the proper curve which the rifled barrels ought to possess in order to give the projectile the necessary degree of rotation; this, in all cases must be sufficient to keep the projectile with its point foremost.
The Enfield rifle, with one turn in 6 feet 6 inches, causes the bullet, on issuing from the muzzle, to rotate once on its axis in six and a-half feet. This moderate degree of rotation only admits of short projectiles being used, as long ones turn over on issuing from the barrel, and at long ranges, the short ones become unsteady.
With the hexagonal barrel, I use much quicker turns, and can fire projectiles of any required length, as with the quickest that may be desirable they do not "strip." I made a short barrel with one turn in the inch (simply to try the effect of an extreme velocity of rotation) and found that I could fire from it mechanically-fitting projectiles made of an alloy of lead and tin, and with a charge of 35 grains of powder they penetrated through 7 inches of elm planks.
At first I was desirous of using, if possible, the turn adopted for the Enfield Rifle, for firing long projectiles, and I tried various shapes and combinations of metal, so as to place the centre of gravity in different positions more or less forward; but when they were fired, the marks left by their passage through a paper screen, placed about 6 feet from the muzzle of the rifle, showed that they all turned over within that distance, because the rotation given by that comparatively slow turn invariably proved insufficient to keep them point foremost. For an ordinary military barrel, 39 inches long, I proposed a .45-inch bore, with one turn in 20 inches, which is, in my opinion, the best for this length. The rotation is sufficient, with a bullet of the requisite specific gravity, for a range of 2,000 yards. The gun responds to every increase of charge, by giving better elevation, from the service charge of 70 grains up to 120 grains; this latter charge is the largest that can be effectively consumed, and the recoil then becomes more than the shoulder can conveniently bear with the weight of the service musket.
The advocates of the slow turn of one in 6 feet 6 inches consider that a quick turn causes so much friction as to impede the progress of the ball to an injurious, and sometimes dangerous degree, and to produce loss of elevation and range; but my experiments show the contrary to be the case. The effect of too quick a turn, as to friction, is felt in the greatest degree when the projectile has attained its highest velocity in the barrel, that is at the muzzle, and is felt in its least degree when the projectile is beginning to move, at the breech. The great strain put upon a gun at the instant of explosion is due, not to the resistance of friction, but to the vis inertiæ of the projectile which has to be overcome. In a long barrel, with an extremely quick turn, the resistance offered to the progress of the projectile as it is urged forward becomes very great at the muzzle, and although moderate charges give good results, the rifle will not respond to increased charges by giving better elevation. If the barrel be cut shorter an increase of charge then improves the elevation.
The use of an increasing or varying turn is obviously injurious, for besides altering the shape of the bullet it causes increased resistance at the muzzle,—the very place where relief is wanted.
Finding that all difficulty arising from length of projectiles is overcome by giving sufficient rotation, and that any weight that may be necessary can be obtained by adding to the length, I adopted for the bullet of the service weight (530 grains) an increased length, and a reduced diameter, and obtained a comparatively low trajectory.[1] This affords very great advantages; less elevation is required and the path of the projectile lies more nearly in a straight line, making it more likely to hit any object of moderate height within range, and rendering mistakes in judging of distances of less moment. The great difficulty experienced in the use of the rifle for long distances arises from the utter incapability of any marksman to judge correctly the distances of objects far away; it can only be met by making the projectile move in a trajectory or path as low and level as possible, this is done by using the reduced diameter which offers to the resistance of the air a proportionally less area, so that a high velocity is attained, shortening thereby the time of flight, and the projectile is very much less deflected by the action of the wind.
While the ordinary grooved rifle depends upon the expansion of the soft metal projectile, in the hexagonal system, rifling may be effected independently of expansion, by making the projectile of the same shape as the interior of the barrel: in other words, by having a mechanical fit between them. The projectile may be used naked, and be made of metal of any degree of hardness. The expansion principle may also be combined with an easy mechanical fit, so that a projectile of metal harder than lead, as an alloy of lead and tin, may be used, which, while it loads easily, will expand sufficiently to fill up the bore, and give more than double the penetration of lead.
This system thus admits of the employment of projectiles of different densities and varied shapes, suitable for special purposes. If for example it be required to fire through plates of iron, a flat-fronted projectile of steel of the form shown in figure E, will be employed. When the flat-fronted projectiles were fired in comparison with those having rounded fronts, like figure B, from the same barrel, and under precisely similar conditions as to range (500 yards), charge, and weight, the pointed projectiles were not found to possess any great practical advantages over the flat ones, for the elevation of the former was only a minute and a-half better than that of the latter.
It was satisfactory to find that so slight a 
difference existed between the two shapes, for in other respects the flat-fronted projectile has many advantages. Besides being the form best adapted, as has been said, for firing against iron plates, it may be used for penetrating through water; in this respect I have succeeded in obtaining results which, while they are most important, were, I believe, previously considered unattainable. For firing through elastic materials, tubular projectiles, as shown in fig. D are used. They are fired with a wooden wad, and take out a core from the object perforated, leaving a corresponding hole which does not close up. The tubular projectiles also penetrate deeper into masonry than those of any other shape I am acquainted with.
The system of rifling which has been described with reference more particularly to small arms, is equally applicable to ordnance of all sizes. The principle of construction is simple and the extent of bearing afforded by the rifling surfaces provides amply for the wear of the interior of the gun; any requisite allowance for windage may be made at the same time that the projectile is kept concentric with the bore. When it enters the gun it bears upon a certain portion of each side of the hexagon, and when it comes out it bears on the opposite portion,—by easing off, therefore, that half of each side on which the projectile bears as it enters, proper windage is allowed,—the other halves of the sides are left to preserve a full extent of bearing surface as it comes out, and at the same time maintain its concentricity with the bore. This plan of providing for windage obviates the great evils which arise from eccentricity in the case of spherical shot.
The effect of a proper rifling turn, which varies according to the bore and length of the piece, produces, as has been before shewn,[2] no loss of elevation while the rotation given to the shot adds greatly to its destructive power.[3] The practice obtained from an ordinary 9-pounder brass field gun, rifled by me, proves that, with a charge of powder one-third less than the service charge, the range was one-half greater than that of the same gun with spherical shot.
In comparing the strain upon a rifled cannon with that upon a smooth bore, the work done by the respective pieces must be taken into consideration. If the full powers of the former are to be exerted, and, if the extreme ranges which it is capable of giving are required, a proportionally greater strain will be put upon the piece on account of the elongated form of the shot, and more strength is therefore required.
The mode of making large iron guns, by casting them in solid masses (as at present), is highly objectionable, and is certainly not suitable for bearing the full strain of a rifled gun. It is well known that if iron be cast in large masses, great irregularities will be produced in the metal during the process of cooling; and, beyond a certain limit, little or no increase of strength is gained by increasing its thickness.
Improved modes of construction can, however, be adopted, which will admit of the gun being loaded at the breech when required, and will give all the strength necessary for as large a charge of powder as can be consumed with the projectile intended for the piece. It is clear that every gun should have that amount of strength as a minimum standard.
In the construction of rifle cannon many things have to be considered in relation, and often in antagonism to each other, as the weight and length of the piece,—the diameter and weight of the projectile, the charge, the amount of rifling turn, and it sometimes happens that an advantage in one respect can only be gained by submitting to disadvantage in another. The amount of turn to be given must be considered with relation to the charges intended to be used and the length of the gun, but should be as quick as possible, since the extent of true steady flight, in the case of a long projectile, depends on the rotation given to it For a great range, length of gun is requisite, as it provides for the consumption of large charges of powder, but the quick turn, which is so desirable, limits the length that can be used. The best combinations and proportions can only be arrived at after careful experiment.
I would observe, before concluding this paper, that it has been drawn up rather with the view of offering some reply to many inquiries, than with the intention of giving a detailed account of what has been done, or of presenting a complete treatise on the subject.
- ↑ The trajectory of a projectile being the curve it describes in its passage; its height is represented by a perpendicular from its highest part let fall on a horizontal plane passing though the rifle.
- ↑ See Ante p. 78. Out of a 24-pounder howitzer, having a turn of one in forty inches, I fired, with low charges, shells 10 diameters in length.
- ↑ When a rifle cannon shot enters into a ship, and strikes at an angle, it is deflected from object to object and sweeps round inside the ship, going through or tearing everything it encounters.