I have now done my best to describe the various types of markings which may be found on a fired cartridge case and their sources of origin, and the next step is to consider how these facts can be utilised properly and correctly in the task of identifying some individual arm.
If a cartridge case is found on the scene of a crime, and a pistol is found later which may have been used by the murderer, the procedure to be adopted in order to determine whether the"crime" cartridge case "marries" the suspect pistol is as follows.
A series of rounds is fired from the suspect pistol the number constituting the series depending on whether any fired bullet has been recovered from the victim or not. If a bullet has been recovered, and it is found to be in sufficiently good condition to use as additional evidence for identification, the first series of rounds should not exceed five, for reasons which will be explained later. This series will presumably be fired by some investigator on behalf of the police. Later on, if a prosecution is decided upon, the defence will have the right of conducting an investigation of their own; in which case a second series of rounds will have to be fired. If bullet evidence is to be used the total number of shots fired through the barrel should not exceed ten, unless the evidence is to be of a very general character. It is for this reason that I have suggested that the first series of rounds should not exceed five if a bullet has been recovered.
The lower right-hand part of the surface of a Fired Lead Revolver Bullet showing the fine Striations of Engraving
But if no bullet has been recovered, or the bullet is too disfigured to be of any use in identification, the number of rounds is immaterial, and the more the better, up to a point, as the evidence may be strengthened.
At least two of these test rounds should be fired with "oiled cases." That is the sides of the cartridge should be well oiled before it is loaded in the chamber. The effect of this oil is to reduce the friction and so reduce the tendency of the case to stick to the inside of the chamber when it is expanded on discharge, and thus increase the force with which the base is driven against the breech face. This naturally means an increase in base pressure which tends to produce a more perfect imprint of the breech face on the base of the cartridge, and so helps the work of detecting the thumb-mark of the weapon.
After every shot the fired case should be carefully collected. This is not always too easy a matter, as some self-loading pistols eject their fired cases to an astonishing distance, while different makes throw the cases in different directions. It is, therefore, a wise plan to cover the hand and breech portion of the pistol with a rag or duster before firing, which will prevent the fired case from being ejected too far and lost.
The bases of all the fired cases should then be examined under a good pocket lens or microscope in order to find out the thumb-mark of the suspect weapon. As has already been explained the thumb-mark of individual weapons will vary in form and size, and I have given a number of typical examples. I suppose that different individuals will naturally work on somewhat different lines, and so I will merely suggest the procedure which personally I find the most convenient, quick and certain.
For many years I have made it a habit to examine a few samples of any lot of fired cartridge cases which I happen to find. It may be a heap on the firing-point of a rifle range, or at some stand in a pheasant covert: I always examine a few with a X 10 aplanatic pocket lens which is my constant companion. I have thus examined many thousands of fired cases, and constant practice enables one to spot markings very quickly.
From 1932 to 1938 I was one of a band of about a dozen riflemen who carried out accuracy trials for an experimental -303 cartridge which contained a special stream-line bullet. We all used at least two rifles each and the tests comprised the firing of long series of shots at 1,100 yards. I made a habit of examining the fired cartridge cases used by the different marksmen, and very soon got to know the individual markings of the various rifles employed. It was, in fact, quite easy to say definitely when shooting had finished which marksman had been firing on which target and which of his rifles he was using merely by examining with a X 10 pocket aplanatic lens the fired cases left on the firing point.
Such a preliminary examination gives some constant and prominent marks which make it possible to orientate all the test cases similarly. I can then stick them on an ordinary glass microscope slip with plasticine, with, of course, their bases upwards. They should be orientated similarly—this can be done quite easily with the pocket lens—and placed in a row as close to each other as possible.
I then examine them under a microscope, beginning with as low a power as possible, usually a 4-inch objective and a X 5 eye-piece. With this low power one can often get three cartridges in the fieid of view at a time, and the illumination is then adjusted so as to fall quite obliquely on the bases. I then slowly turn the cartridges through 180 degrees by means of the revolving stage of the microscope. This ensures the light striking them at every angle, and in the course of the rotation a point will come when some mark or marks suddenly shows up very distinctly. When this point is reached I look for this mark in all the cartridges which are in the field of view, and if it is present in all I bring the rest of the test cases into the field one by one by traversing with the mechanical stage, taking care always to keep a cartridge which has already been examined in the same field as a new cartridge, so as to ensure getting a guide in every case.
In revolver cartridges there will be no ejector mark, but orientation can be obtained just as well by the stria-tions on the cap, that is by fixing the cases so that all the cap striations are running in the same direction.
If there are no very pronounced and isolated marks common to all the test cartridges it is a help to use a higher power of magnification, and a 3-inch objective can be substituted for the 4-inch. Even with one of these objectives it is possible to get the bases of two 38 revolver cartridges in the same field simultaneously.
With this power a more detailed comparison can be made of the finer markings, particularly the cap striations, and there is seldom much difficulty in finding several characteristics in the markings which are common to all the test cartridges. Once these characteristics have been discovered the thumb-mark of the suspect weapon is known, and the next step is to examine the breech face of that weapon so as to check the thumb-mark. This can usually be done quite well with a good pocket lens, but a more leisurely examination can be made with a microscope if it is possible to fix the breech face on the instrument.
Having now settled the question of the thumb-mark beyond all doubt it but remains to examine the "crime" cartridge to see whether the thumb-mark is also reproduced on its base. This can be done most simply by placing the "crime" cartridge and a well-marked test cartridge side by side on a glass slip, orientating them similarly under a pocket lens by means of the ejector marks, cap striations, or any other convenient feature which may seem to be common to both. The two cartridges can then be examined in the same field of the microscope, when identity or difference should readily be established.
Sometimes, when the markings are very fine, it may be advantageous to use a higher power than the 3-inch, and then a 2-inch objective can be employed. With this the field is too limited to accommodate more than one small-sized cartridge, or the part of a larger one, at a time; but by traversing with the mechanical stage the cartridges on the slip can be examined in turn and their similarities or differences easily detected.
A 2-inch objective may also be occasionally useful to determine the presence of some minor marking in a low-pressure round in which the imprint is not very clear. But even so the occasions on which it is required are few and far between, and a 3-inch objective will be found by far the most generally useful.
A higher power than a 2-inch should never be required for cartridge cases provided the illumination is correct. For illumination has far more to do with rendering detail visible than has magnification. But I propose to deal with this subject later on.
The sides of the cartridges can be examined by mounting them on a rotating spindle with plasticine at right angles to the axis of the microscope. The illumination is focussed obliquely on one edge of the side and this edge also
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