Modern rifles, revolvers, and pistols have barrels which are „rifled," i.e., they have spiral grooves in the inner surface, the purpose of which is to cause the bullet to acquire a rapid spin on its longitudinal axis, the gyroscopic effect of which keeps the bullet from „yawing" or „tumbling" in flight. This method of improving the accuracy of the flight of a bullet has been used for hundreds of years and no one knows just when the principle was first discovered. These grooves in the bore of the barrel and the lands (ridges) between them constitute the rifling. In present-day hand guns the pitch (or twist, as it is called) of the rifling is uniform from one end of the barrel to the other. Many years ago certain manufacturers used a „gain twist" in which the angle of twist increased from breech to muzzle. This will be discussed in a later section.
While the method whereby the grooves were first produced seems not to be definitely known, it can be said that until fairly recently there have been two methods in general use for producing rifling. These methods are the „scrape cutter" method and the „hook cutter" method. Most of the weapons the firearms examiner will encounter have been rifled by one of these two methods, and mostly by the second. However, this situation will change as many firearms now being made are being rifled by newer methods which have the advantage of being more rapid.
A typical scraping device is shown in Figure 1. It consists of a rod, slightly smaller than the bore of the gun, into which is set either one or two curved, hardened steel scrapers the height of which can be adjusted between successive passages through the barrel. If an odd number of grooves is to be cut, a single scraper is used. If an even number, two scrapers placed opposite each other may be used. Of course an even number can be cut with a single scraper, one at a time. The operation is a very slow one, particularly if five or six grooves are to be formed by a single cutter. Some of the finest rifling ever done has been done by the scrape-cutter method.
In European practice there seems to have been more variation in the application of the scrape-cutter 4 method. It seems that the following variations of rifling head construction have been used: (1) Two single curved cutting blades, each of which is an integral part of the surface of a plate that fits closely into slots that are placed opposite each other in the rifling head, so that two grooves are cut simultaneously. (2) Four cutting blades, two on each plate set „tandem" to each other so that the second cutter follows in the groove made by the first one, thus deepening the groove, the plates being set into slots opposite each other in the rifling head. Thus two opposite grooves are cut simultaneously but deeper than in (1). There is evidence that in some cases three cutters were set in tandem (in each of the two oppositely placed plates) so as to increase still more the speed of the rifling operation. (3) Two cutters on each of two plates set opposite to each other, but instead of being set in tandem they are set apart (with respect to the longitudinal axis of the rifling tool) a distance equal to the desired width of the land. Thus, when this assembly is pulled through the barrel four equally spaced grooves are cut simultaneously. (4) Three cutters arranged symmetrically in the rifling head so as to cut three equally spaced grooves simultaneously. To get the second set of three grooves the barrel or the rifling head is indexed into the proper position so that the completed job will have six equally spaced grooves.
In each of the above procedures a wedge-shaped rod (or shim) is pushed in a bit automatically at each rifling stroke so that for each passage the cutters are raised the desired amount, the process being repeated until the desired depth of groove is attained. The cutting edges in each case above are formed by milling away the steel of the plate so as to leave the curved cutter with the desired shape, height, and angle.
In the „hook cutter" method, a cutter with the general shape of a crochet hook (Fig. 2) is set into a recess or slot in a rod (Fig. 3) which is a bit smaller than the bore of the barrel. The height of the cutting edge of the „hook" can be adjusted by turning an adjusting screw at the end of the rod. On each pass through the barrel a fraction of a thousandth of an inch of steel is removed, and as the barrel is given a steady rotation at a predetermined rate a very shallow spiral groove is formed, having the width of the cutting edge. The barrel is then positioned so as to cut a second shallow groove parallel to the first one, and this process is repeated until all of the grooves have been started. Since they are not yet of the desired depth the cutting edge of the „hook" is now raised a bit and each groove is cut a bit deeper. The process is repeated again and again until all the grooves have been cut to the desired depth-amounting to a few thousandths of an inch. In the cheaper guns only 20 or so passes are used for making each groove, whereas on a finely made firearm as many as 80 or more might be used. Here again the process is exacting and time consuming, and for these reasons newer methods of rifling are coming into general use. In theory each groove is cut to very exact dimensions. All lands are supposed to be equal in width and all grooves are supposed to be of equal width and depth, but actually this perfection is never obtained and some manufacturers pay little attention to specifications.
Whether a scrape cutter or a hook cutter is used, a microscopic examination with sufficient magnification of the cutting edge would reveal the fact that the edge is not truly smooth. It would have nicks in it, just as the blade of a dull knife has them, the only difference being that they are smaller. No matter how much care is used in the honing operation nicks will still be present and they result in serrations or ridges being formed in the bottom of the groove made by the cutter.
It must be remembered, also, that the steel used in barrels is not absolutely homogeneous and there will be some areas of the surface which are harder than others. The cutter will not act the same on these areas with different hardnesses and this will result in inequalities in the surface. Also, tiny chips of metal from the cutting operation may produce inequalities in the action of the cutter. No matter how these inequalities are produced, unless they are removed they will tend to make repetitive marks on bullets because the bullets are of softer material. Various methods are used to remove these inequalities.
Some manufacturers perform a „lapping" operation after all the grooves have been cut. A lead plug is cast on the end of a rod placed in the barrel. This, of course, insures a good fit. Then, with a mixture of oil and fine emery powder as a lubricant and polishing agent, the plug is pushed back and forth through the barrel. Finally a mirror-like surface is produced and most of the inequalities in the surface, produced in the boring, reaming, and rifling processes, are removed. A new, lapped barrel will leave fewer and smaller markings on a bullet fired through it than will an unlapped one, but marks will still be present and identification will normally be possible.
Next to be mentioned is the broaching process which is now used by several American firearms manufacturers and some in Europe as well. As somewhat of an oversimplification, a broach may be thought of as a rod upon which there are 25 to 30 hardened steel rings, each one being slightly greater in diameter than the preceding one and having slots of the proper size cut into it at equal intervals, thus forming a series (or „gang") of cutters, each of which has the same number of lands and grooves. (Figs. 4, 5) The lands on the cutters produce grooves in the barrel and all grooves are cut to the desired depth during a single passage of the series or gang of cutters through the barrel. Each successive cutter must be perfectly aligned so that its lands will follow the grooves made by the preceding (smaller) cutter. This is a simplified description of the process but will serve to illustrate the principle.
The broaches require much skill in their preparation but each broach is capable of rifling a large number of barrels and the operation time has been reduced to such an extent that one machine can rifle several hundred barrels in a working day. Also, many more can be rifled with a broach than with a single scrape cutter or hook cutter because the broach has many more cutting edges and the wear on each edge is consequently less than that on a single cutter.
It might be thought that since many barrels are rifled with the same broach, all the barrels would have lands and grooves which are exactly alike and that it would be impossible for the firearms examiner to distinguish between bullets fired from them. This, however, is not the case. Each rifled barrel still possesses an individuality which is expressed in the markings made on bullets fired through it. This is due in considerable part to the fact that there is one thing in common in all three of the methods of rifling so far discussed. In all cases the preparation of the bore of the barrel to be rifled is essentially the same. A hole of suitable diameter is bored from end to end through the piece of stock that is to become the barrel. Since the surface so produced is too rough it has to be reamed in order to smooth it sufficiently. In this process of reaming, the movement is transverse to the axis of the barrel and if a cross section of the barrel is examined microscopically at this point a multitude of small lines (scorings) running crosswise of the bore will be observed. In the succeeding rifling operation a portion only of these transverse lines will be removed, i.e., in the areas where the grooves are cut. They will still be present on the lands. If a cross section of a rifled barrel is examined (Fig. 6) it will be seen that there are two sets of lines or tiny scratches, those on the lands running transversely and another set at the bottom of the grooves running longitudinally. There will frequently be defects in the surface of the lands in the barrel due to scoring by chips produced in the broaching operation, and it appears that the broach has a tendency to „ride the lands" and produce changes thereon. Any inequality in the surface of the barrel with which the bullet comes in contact may produce a scratch or serration on a bullet fired through it, and since the greater pressure against the bullet is produced by the lands on the barrel the most prominent markings are likely to be found at the bottom of the grooves on the bullet. However, if the bullet fits quite tightly longitudinal striations will be found on the lands of the bullet also, caused by the scraping of the bullet along the bottom of the grooves of the rifling. In many of the more cheaply made guns, particularly those made in the 1910-1935 period in Spain and Belgium, the reaming operation apparently was omitted, or was so poorly done that it might just as well have been omitted. Many of the cheaply made American guns also belong in this category. Since at one time an American revolver could be purchased for $1.50 one could not expect much! Unfortunately many of these are still in circulation.
Another system for rifling barrels is fast coming into favor and some believe that it may eventually replace completely the broaching process as it should produce a barrel in which the rifling will have a longer life since it causes a hardening of the surface which comes into contact with the bullet. The process seems to have been used first in Germany but has been the subject of considerable experimentation in the U.S. and is in use by at least two manufacturers. It is also being used in other countries.
This process is known as the swaging method. When a plug of extreme hardness (called a „button") is forced through a barrel the bore of which is slightly smaller than the button, the metal of the barrel flows slightly under this very high pressure and the bore is slightly increased (Fig. 7). Because of the elasticity of the metal of the barrel the bore will not be quite as great in diameter as the button itself, but it will be greater than it was before the swaging operation. If the button has a very smooth surface and is very hard, the newly produced bore will be very smooth, of very uniform diameter, and it will have a harder surface because of the increased density of the metal produced by the compression it has undergone. This is said to be superior to the reaming operation, particularly as the latter is usually performed. The grooves are formed in a similar operation, but with a torpedo-shaped button made of tungsten carbide, or other similar material of extreme hardness, which is provided with lands and grooves that are the negative of those to be produced in the barrel. As this passes through the barrel it causes a further compressing of the steel as well as producing rifling grooves. If both button reaming and button rifling procedures are used all of the interior surfaces of the barrel will be hardened and all will be exceptionally smooth. Any striations present, either on the lands or in the grooves, will run longitudinally through the barrel (Figs. 7, 8). Steels which are so hard that they give trouble when other rifling methods are used can be rifled by the swaging method. The making of a suitable button requires great skill because of the extreme hardness of the material and because it must have very precise dimensions to function properly; but once formed the same button may be used for the rifling of thousands of barrels. Because of the extreme hardness of the material, diamond-grinding processes must be used in forming them.
Because of less wear on the cutting edges in the broach and button methods, the lands and grooves in barrels rifled successively by one of these methods will be more alike than those in barrels rifled by the scrape-cutter or hook-cutter method. Indeed, many of the major markings (scorings, striations, etc.-particularly along the edges of the grooves) may be repeated from barrel to barrel, and the widths of the lands and grooves, though practically never exactly the same in a barrel, will show the same sequence of variations in barrels rifled by the same broach or button. Consequently, the examiner must be on guard, as it is then no longer possible to rely either upon agreeing sequences of land or groove widths or on the recurrence of striations or gouges on the edges of the grooves on a fired bullet. He must resort to closer examinations, often with higher magnifications than formerly necessary. Fortunately for the examiner, barrels rifled by these newer methods (and having nearly identical characteristics) still do have individuality, and as the gun is used (and abused) each barrel will develop more individuality.
Still another method of producing rifling, though it is doubtful whether it will come into general use for firearms of good quality, is also a swaging method, but a quite different one. In this method a tightly fitting mandrel of very hard steel, bearing a negative form of the rifling desired, is pushed into the bore of the barrel after the reaming operation and the barrel is then compressed onto the mandrel under very high pressure so that the steel flows into the grooves in the mandrel, filling them completely, thus forming a set of lands and grooves in the barrel. This method was used to some extent during World War II in the production of the M-3 submachine gun. Hard steel, such as is used in high powered rifles and good grades of hand guns, does not lend itself to this process, and the method has many difficulties.
The removal of the mandrel from the barrel appears to be a tricky job and during its removal the rifling is likely to be damaged somewhat. Here again experience has shown that every barrel produced has an individuality and bullets fired from different barrels can be easily distinguished.
To increase the rate of production of rifled barrels a cold forming process, known as the „hammer" process, was developed in Europe by a Dr. Appel, and this process has been introduced into the United States since World War II. It is being used by Appel Process, Inc., in Detroit, Michigan.
In this method a steel tube is passed over a short mandrel, composed of very hard steel, which bears a negative impression of the rifling desired. As the tube advances on the mandrel, multiple hammers pound the metal of the tube into the grooves of the mandrel. The degree of twist of the rifling so produced is determined by the pitch of the lands and grooves on the mandrel. The perfection of the rifling will depend on the perfection of the mandrel and on how perfectly the grooves in the mandrel are filled.
This is a swaging process but it differs from the one previously described in that the metal is caused to flow under the pressure produced by hammers, in an automatic machine, rather than by pressing a tube onto a mandrel by the application of a very high pressure, applied more uniformly. It would appear that the process would have some of the disadvantages experienced in the preceding process, notably the necessity of using a soft, malleable steel which would be quite unsuited for use in any but the cheaper, small-caliber arms. It doubtless has one decided advantage over the preceding process in that the mandrel used is relatively short and the difficulty of removing the long mandrel from the completed rifled tube would be eliminated.
A very curious type of rifling, which has been encountered only in the Galand revolver, is that shown in Fig. 9. How this was produced is not known. The twelve convex lands would not seem to be as effective as the rifling generally used.
Another unusual type of rifling is that which was used in the .22 cal. single-shot Hamilton rifle, which was patented by Clarence J. Hamilton and Coelle Hamilton, U.S. Patent No. 600,725, dated October 30, 1900.
In the completed barrel the bore is a dodecagon, i.e., the rifling consists of twelve chords approximately equal in length, as shown in Fig. 10. Actually the chords vary-in length (in one specimen at least) from 0.046 to 0.052 inch, though presumably they were intended to be equal. In the usual sense there are no lands and grooves.
This peculiar rifling was produced by slipping a seamless brass tube tightly over a steel mandrel which had the negative profile of the desired shape and subjecting the tube to powerful pressure in a suitable press. This compressed the brass into the desired shape and, according to the patent, „hardens the brass" due to the considerable pressure applied. After the brass tube had been pressed into its new shape a steel tube was sweated on to give added strength. In a variation of procedure, a heavier brass tube was taken and pressed into the desired shape, and no steel tube was used. The first procedure was preferred, however. Barrels longer than the mandrel were produced by taking a longer brass tube and pressing it in sections, being careful to have some overlap so that the bore would be uniform.
The gun was produced from about 1902 to 1908, and apparently in several different models. In Mod. 15 the barrel is but 8 inches in length although the over-all length is 271/2 inches, the bolt being actually longer than the barrel. While the gun is only of historical interest, the manner of producing the rifling is of special interest as it seems to have been a forerunner of one of the modern procedures used in rifling gun barrels.
Fig. 1. Section of a „scrape" cutter. For rifling barrels with an even number of grooves, two (opposite) scrapers were used.
Fig. 2. Four „hooks" used in the hook cutter method of rifling a barrel.
The cutting edge of the hook, which has been filed to exact dimensions, projects through a slot in a rod whose diameter is slightly less than that of the bore to be rifled. The height to which the cutting edge protrudes is adjusted by a screw at the end of the rod.
The rod is pulled through the bore and on each pass a very small amount of metal is removed. Each groove is started with the same adjustment of the cutter, After all have been started the cutting edge is set up a bit and the process repeated on all grooves. This is done again and again until all the grooves are cut to the proper depth. This may take as many as 80 passes for each groove in the rifling of a first-class gun.
Since the hooks wear away during the cutting operation they have to be replaced frequently to keep within the tolerances that have been set in the specifications. Some manufacturers pay little attention to their „specifications," however.
The hooks shown are for .25, .32, .38, and .45 calibers.
Fig. 3. The hook cutter. Top: Section of cutter for .22 caliber. Middle: Enlarged view of hook section. Bottom: Enlarged view of adjusting screw.
Fig. 4. Broach for cutting rifling. Upper view shows the entire tool, about 23" in length, with 27 cutters. The lower view shows a section enlarged.
Fig. 5. Broaches for cutting rifling. Top views show three entire broaches: A .38 cal. Colt broach and two H & R broaches, all 16 inches long and having 25 cutters.
Enlarged views of short sections of the Colt broach and one of the H & R broaches are shown in the lower part of the figure.
Fig. 6. Section of barrel rifled with a „hook" cutter. Longitudinal marks due to rifling tool. Transverse marks due to drill.
Fig. 7. Rifling buttons. Upper: Bore button swage. Lower: Groove button swage.
Fig. 8. Rifling button used for .22 cal. rifles.
Fig. 9. An unusual type of rifling. 12 rounded lands. Galand revolver.
Fig. 10. Unusual type of rifling used in .22 cal. Hamilton rifle, Mod. 15. Bore is a dodecagon in cross section. Rifled sleeve is of bronze.
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