Cartridges, especially those fired in automatic or repeating firearms, often show repetitive marks which are useful in the identification of the type (and sometimes the make) of weapon used and of the individual arm when test cartridges fired in it are compared with an evidence cartridge. Impressions that are made by file marks, tool marks, or other inequalities on the surface of the breechblock when a shell sets back against it under high pressure are likely to be more reproducible than marks made by the sliding of a bullet over a slightly rough surface, such as the interior of a rifled barrel. The former resembles the process of printing from fixed type, or that of die stamping, whereas the markings on bullets are made by a process more like that of making tool marks where the tool slides while in contact with the surface.
The size, shape, and location of extractor and ejector marks, the type of breechblock mark, the presence of magazine scratches, etc. are all important in helping to determine the type and make (and possibly the model) of a gun used. In the case of rim fire ammunition, the size, shape, and location of the firing pin impression is of value in determining the make of arm used. This will be discussed later.
The individual weapon in which a cartridge was fired can often be determined by comparison of the markings on the evidence shell with those produced by the suspect arm on test shells. Here again it is important that the same make of ammunition be used as that in the evidence submitted. In fact it is even more important here than in the case of bullet comparisons, because small differences in thickness or in the composition of the primer or in the composition of the brass of the shell head will make marked differences in the distinctness of the marks produced.
In many cases ammunition of the same make as that of the evidence shell will be found in the possession of the suspect. These cartridges are the ones that are preferably used to secure test shells, as they are more likely to have been made from the same batches of metal (i.e., primer and case). Not all ammunition of the same make will be made of metals having the same degree of hardness, and one cannot say definitely that the confiscated cartridges were made of metals from the same batches as those used in making the evidence shell, although the probability is great that such was the case. Therefore, if reproducible and matching marks are not present on test shells it is quite proper to use other cartridges of the same or even of different makes of ammunition, because if the marks on the evidence shell can be reproduced on any ammunition it is sufficient proof that one has the right gun. Naturally it is more satisfying if one can testify in court that he used ammunition of the same make as that of the evidence shell and found in possession of the suspect.
Sometimes primers have protective coatings and if the evidence shall has such a coating a cartridge having a primer similarly coated should be used. Frequently good, well-defined marks present on an evidence shell will not be present on a test shell or, if present, may be poorly defined when a different make of ammunition is used. This may be due to differences in the primer or it may also be due to differences in powder pressures. The higher the pressure the more distinct the marks are likely to be. Since many of the cartridges used in crime cases were made years ago it is imperative that the firearms examiner have available a collection of old ammunition as well as of that currently in production. Rim fire firing pin marks
The most distinguishing mark to be found on a rim fire shell is the impression of the firing pin. Breechblock marks are almost never present, unless the new „magnum" cartridge is used. Extractor marks may be present (although not necessarily so) on shells that are ejected in the process of firing. As these are the ones most likely to be found at the scene of a shooting they are naturally the ones in which we are most interested. Occasionally extractor marks are sufficiently repetitive and distinctive in character to permit matching under the comparison microscope, consequently they should always be looked for. Ejector marks are less likely to be found, and when present they are rarely amenable to profitable comparison with the microscope. However, the position of the ejector mark and the extractor mark (if such marks be present) with respect to the location of the firing pin impression is important and will be discussed later.
Frequently the details of a firing pin impression will show characteristic features which are repetitive, and in such cases matchings will be possible. These comparisons are usually made with the comparison microscope but can also (though less conveniently) be made with the comparison camera; and with either instrument photographic recordings of what one sees can be made. Figs. 19 and 20 show examples of such comparisons.
The term „firing pin" as used here includes the various devices which are used to cause the shell to fire. In some cases the firing pin is an integral part of a hammer, in others it may be a separate entity hinged to the face of the hammer, or it may be an actual pin or rod (spring loaded or not) driven forward by a spring-actuated hammer or some other spring-actuated system. In any case the end of the „firing pin" strikes the edge of the shell causing a detonation which sets fire to the powder, and in so doing it leaves a more or less perfect facsimile impression of the face of the pin on or near the rim of the shell. These impressions are often excellent.
Fortunately for the investigator, different manufacturers have different ideas concerning the best shape and size for the end of the pin. As long as each manufacturer sticks to a definite form and size of pin for each model of gun he produces, and many do quite consistently over a considerable period, the firing pin impression is an important characteristic. If frequent changes are made its value is lessened. As long as it remains unchanged it may help the examiner to (1) give police officers some idea as to what make or makes of guns are possibilities in the case and (2), in cases where guns are submitted for examination, it may help to eliminate a suspect gun or guns. In one instance in the author's experience eleven .22 cal. repeating rifles were submitted for examination in connection with a shooting case.
All eleven could quickly be eliminated because none of them produced the characteristic firing pin impression found on the evidence shells.
Since there are fairly distinct shapes of firing pin impressions they may be classified as to type. The H. P. White Laboratory, for example, uses the following classification: Bar (extending across shell head), Rectangular, Round, Semicircular, and Special. In addition, the approximate vertical and horizontal dimensions are noted. Still other classifications are possible, such as: Bar, Rectangular (Square), Rectangular (Narrow), Rectangular (Broad), Round (Large), Round (Small), Semicircular (Large) , Semicircular (Small), and Special. The term Special is used for those which do not conform to any of the previous types, and, as a matter of fact, they are comparatively rare.
With respect to classifications such as the foregoing, some words of caution are necessary. If all of the marks made by a given firing pin were always alike as to shape and dimensions, the situation would be quite simple, but, unfortunately, this ideal situation does not exist. If a new clean gun is used, with the same ammunition, and if the shell is properly seated in the chamber, the marks should be quite repetitive in shape, location, and dimensions. In a much-used gun, or in one poorly made, the pin may not strike in exactly the same place (Fig. 21). The length of a rectangular firing pin impression will thus be affected, and a round-faced pin could produce either round or semicircular impressions of different sizes (Fig. 22). The size, and to some extent the shape, of a firing pin impression will depend on the depth of penetration of the pin (Fig. 23). Frequently a cartridge at the moment of firing is not seated firmly against the shoulder of the chamber, and „normal" depth of penetration will not be achieved. This obviously will affect the dimensions of the impression, be the pin of the round-end or rectangular-faced type. The latter are usually somewhat wedge shaped (to add strength) ; consequently, a deeper impression will be both wider and longer. Firing pins having rounded ends or flat circular ends are frequently tapered and here again the size of the impression will depend on the depth of penetration. If the cartridge does not fit the chamber snugly the point of impact of the firing pin on successive shots fired will not necessarily be the same, and the size (particularly the vertical dimension) of the impressions will not be uniform. A pin having a round (circular) face may produce either . a circular or semicircular impression depending on how near it strikes the edge of the rim of the shell. Most semicircular impressions are made by pins having rounded or flat circular faces.
Another factor of importance is the ammunition used. Firing pin impressions on shells of different makes may show distinct differences. It is well known that even pure copper does not always have the same degree of hardness because of different treatments to which it may have been subjected. Copper from different sources may have different degrees of hardness due to small impurities. Furthermore, the depth of penetration depends, on the thickness of the metal in the head of the shell, and this may vary from manufacturer to manufacturer.
In making comparisons of firing pin impressions on test and evidence shells it is therefore highly recommended that, whenever possible, the same ammunition be used (from the same lot when possible). It is also apparent from what has been said above that several shells should be fired for comparisons. It is recommended that at least three be fired and compared with each other. If the impressions on these are all alike it may be assumed that the evidence shell should show the same marking if actually fired from the gun in question. If significant variations appear in the firing pin impressions on the three test shells more test shells should then be fired. If, after comparing a considerable number, none are found with markings that closely resemble the evidence shell marking one can safely conclude that he does not have the right weapon.
As has been mentioned, the locations of the extractor and ejector marks may be of considerable importance. Just to cite one example, suppose we had to distinguish between two guns-a Winchester Model 74 Self-loading rifle and a Winchester Model 75 Bolt Action rifle, both of which are possibilities in our theoretical case. The information given by the firing pin impressions alone is not conclusive, but there are important differences with respect to extractor and ejector markings. If the Mod. 74 test shell is oriented so that the firing pin impression is at the 12 o'clock position, the extractor mark will be at 3 o'clock and the ejector mark at 9 o'clock. If the test shell from the Mod. 75 is similarly oriented, with the firing pin impression at 12 o'clock, two extractor marks should be present, one at 3 o'clock and the other at 9 o'clock, and the ejector mark will be at about 6 o'clock. A comparison of the evidence shell with the test shells should enable one to determine which of the two guns (if either) may have fired the evidence shell. If the evidence shell shows two extractor marks and an ejector mark at the 6 o'clock position we can safely eliminate the Mod. 74 if none of a series of test shells show these markings. Of course, this does not prove that this particular Mod. 75 rifle was used.
A file of photographs of rim fire firing pin impressions, containing as extensive a coverage as possible, is an important asset to a crime investigation laboratory. While drawings and reproduced photographs are useful, original photographs are preferable. These should show the shell heads at least 1 1/2 inches in diameter in order that the details of impressions may be sufficiently distinct. In taking such photographs proper lighting is essential. The author prefers to use the large Silvermann illuminator, which consists of a circular tungsten-filament lamp (about 21/2 inches in diameter) set into a circular reflector through which the reflected light passes from the shell head to the camera lens. This gives a very even illumination without shadows.
Because of possible variations in the appearance of firing pin impressions made by the same pin, it is recommended that, in building up such a file, three shells be fired and photographed in all cases. If just one shell is fired and photographed one never knows whether successive marks will be repetitive or not, and very erroneous conclusions might be reached. It is most important to know whether the markings are approximately repetitive or not. The binocular microscope is useful in making these tests.
In Appendix No. VIII there will be found several plates illustrating the firing pin impressions made by a number of different makes of .22 cal. arms. Plates Nos. 8 to 14 are made up of reproductions taken from Firearms Information Service cards and are used here by permission.*
*Firearms Information Service was issued monthly by the H. P. White Laboratory of Belair, Md., in the form of 4X6-inch cards containing a wide variety of information in the firearms field which is particularly useful to the firearms examiner and also of interest to the collector. Unfortunately this service has been discontinued, temporarily we hope.
The reproductions shown in Plates Nos. 15 to 27| are from photographs taken by the author. Because of the hundreds of makes and models of .22 cal. rim fire arms that have been made it is obviously impossible to cover more than a small fraction of them. Those here reproduced are typical and it is hoped that they are sufficiently inclusive to be useful. An examination of the reproductions in this Appendix will disclose the fact that different manufacturers use firing pins which make impressions that are very much alike and also the fact that a manufacturer may make changes from time to time. Sometimes these differences (particularly minor dimensional changes) are due to nonuniformity in shop practice and at other times deliberate major changes are made. In spite of the difficulties and uncertainties involved, a study of firing pin impression types is, nevertheless, often a great aid (1) in determining the type or make of a gun that may have fired an evidence shell and (2) in eliminating a large number of arms that could not have fired that shell. And, finally, it must be remembered that frequently the markings on evidence and test shells may be satisfactorily matched so that the identity of the particular gun that fired a shell can be determined.
|In these photographs the orientation of the shells is the same (within close limits) as when they were in the gun at the moment of firing. In some cases a dot of ink was placed on the rim of the shell, to use as a reference point in determining the position of the shell, while in others the shell was oriented with respect to the head stamp.
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