When a bullet, either plain lead or jacketed, passes through a rifled barrel under high pressure, the bullet tends to expand and fill the whole cross section of the barrel. The bullet should be of such size that it does this, in order to follow the rifling as it should, to prevent loss of pressure, and to prevent erosion of the inner surfaces of the barrel by escaping gases. The more completely the bullet fills the cross section of the barrel the more distinctive the markings will be and the better the matchings of rifling marks. Inequalities in the steel surfaces of both lands and grooves will then score the bullet as it passes through the barrel under pressure, and it will show useful markings not only in the grooves (made by the lands of the barrel) but also on the lands, where the pressure was less. The ideal case is that in which all grooves and lands on the fired bullet have markings which show an individuality which is repetitive, i.e., will be found on all bullets fired from this barrel, so that every groove and land can be „matched." Needless to say such perfection is rarely metl In some cases the examiner will be lucky, because of mutilation o£ the bullet, to get one match that is sufficiently convincing to enable him to express a positive opinion. In the case of jacketed bullets the situation where all lands and grooves can be matched is very rarely met because, the greater hardness of the jacketing material prevents the metal from being forced into the grooves sufficiently to fill the whole cross section. Indeed, it frequently happens that the lands of the bullet will show few if any markings of sufficiently repetitive character to enable one to get a matching that carries any conviction.
A good example of bullet markings (at least about as good an example as one can ordinarily hope to get) is shown in Fig. 11.* Such matchings of markings on both grooves and lands is not to be expected in the case of jacketed bullets, and rarely even on lead bullets. From what has been said, however, it must not be inferred that bullet matchings on lead bullets are superior in quality to those to be found on jacketed bullets. As a matter of fact the markings on jacketed bullets (generally in the grooves, however) are usually of better quality than those produced on lead bullets because the jacket is of harder metal.
*Many other examples of bullet matchings will be shown later.
Because of the greater hardness of the jacket, fine engravings produced in the passage of the bullet through the barrel are less likely to be wiped off. But, if the bullet is too small it may not follow the rifling sufficiently to produce repetitive markings. Obviously if the bullet slips there will be a confusion of markings, as two bullets are not likely to slip in the same manner. (A case in point is one where a man was killed by a .30-30 bullet fired in a .32 cal. rifle. Repetitive marks on test bullets could not be obtained. Fortunately, however, the case was solved by comparison of the markings on the head of the evidence shell with those made on test shells.) If a bullet is only slightly smaller than it should be, it may skid as it enters the rifled part of the bore of an automatic and then settle down and follow the rifling. In such a case the marks may be of the same character as the skid marks found on bullets fired from revolvers. Very frequently a bullet, particularly a jacketed one, while showing good markings in the grooves will show little or no marking on the lands because the bullet did not expand sufficiently or was not of proper size in the first place.
In addition to the markings made by the rifling, marks may be made when the bullet strikes the forcing cone (if one is present) and by irregularities that exist at the muzzle. Some foreign-made guns have no chamfering of the rifling or forcing cone and the rifling begins abruptly. The diameter of the bore of the rifled section is less than that just preceding it. The ends of the lands, being sharp (not chamfered), dig into the bullet as it strikes them. In some makes of guns the barrel is constricted at the muzzle, probably caused in the operation of producing the crown. This constriction has an important bearing on the marks that have previously been put on the bullet as it passed through the major portion of the barrel. Previously made markings may be removed and new ones put on. Barrels are frequently found to be „burred" at the muzzle due to accident or some misuse and these burrs may dig into the bullet, producing distinctive marks in the form of longitudinal gouges. When such gouges are found one should examine the muzzle for the presence of burrs.
Frequently the examiner will find a gun that has a bulge in the barrel. These bulges are caused by firing a bullet through a barrel which has an obstruction in it-usually a bullet that has lodged. When a bullet is fired through a bulged barrel it will have two sets of rifling marks which, being out of phase because of failure to follow the rifling through the bulged zone, will be superimposed on each other to some extent and may cause considerable confusion.
When a bullet is fired from a revolver it usually will show slippage or „skid" marks, the grooves being wider at the nose end of the bullet than at the base end. This skidding of the bullet occurs when the bullet strikes the lands of the barrel after acquiring a high velocity during passage from the cartridge to the rifling. Because of the high inertia it has thus acquired, it resists the attempts of the lands to cause it to take a rotational motion; hence, it skids.
Skidding seldom is observed on bullets fired from automatic pistols. It naturally occurs most prominently where the bullet is traveling at a high velocity when it hits the lands. But this does not happen in an automatic, since the bullet, before firing, is practically in contact with the lands. Therefore, it starts into the rifling with comparatively little inertia and follows the lands from the start.
Revolvers which are poorly made or those which are very much worn may have cylinders which are not properly in alignment with the bore of the barrel and in such cases there will be „shaving of lead." An example of this is shown in Fig. 70. This shaving of lead may or may not cause difficulty in identification of a gun. If the performance is repetitive there will be no difficulty, but if it is not repetitive, of course, there will be. If the cylinder is very loose the alignment with the bore of the barrel will be capricious and trouble will be caused. Usually the difficulty is overcome by firing many test bullets instead of the customary three.
Nonrepetitive markings of another type are likely to be encountered and care must be taken that confusion is not caused by their presence. These are tiny scratches, parallel to the axis of the bullet, produced when the bullet is forced out of a shell into which it has been held by the crimping of the mouth of the shell into a cannelure on the bullet, or by points of the shell casing having been peened into the bullet, or even by a simple tight fit of the bullet in the mouth of the shell. These may exist in great number and occasionally some of them may not be removed by the passage of the bullet through the barrel. Since they are so short it may be difficult to determine the fact that they are parallel to the axis of the bullet rather than to the marks made by the rifling and, consequently, they can be mistaken for rifling marks. A bullet which has been forcibly pulled from a shell will, of course, show these marks.
In some of the cheap foreign guns, particularly those made in Spain and Belgium before the Spanish Revolution and World War II, the rifling was very poorly done. Not only were there transverse scrapings on the top of the lands but there were also deep scratches or gouges in the rifling grooves, caused by using cutters which had nicks in their cutting edges. If such gouges are present in a barrel they will produce markings on bullets fired through that barrel. If the nicks in the cutter are rather prominent, several almost identical gouges may be produced in each barrel groove, and a bullet fired through such a barrel might have several very similar markings on each land. When this occurs, pseudo or illusory matchings of one bullet against another could be obtained, i.e., one land might be matched with several others on the same bullet, provided other differentiating markings were absent. Fortunately each groove and land in a rifled barrel usually does have sufficient individuality to prevent an experienced examiner from going astray.
Short barrels can be, and sometimes are, made by cutting the required length from a longer rifled barrel, and the U.S. 45 once had barrels made in pairs which were then cut apart to form single barrels. In the former case the barrels will have the same class characteristics but each will have an individuality which expresses itself on bullets fired through it. In the case of the U.S. 45 the two barrels will have the same class characteristics, even though they are reversed when completed. Each will have a high individuality because the inequalities produced in the processes will also be reversed in order.
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