t is a well-known fact that at the moment when the cartridge is fired, the empty shell is hurled violently against the breech face, as a result of the recoil. The primer and shell head receive certain imprints from the breech face, differing somewhat according to the pressure developed within the individual cartridge (Fig. 284). These impressions vary considerably in their nature, and may be of great value in determining the type or even the particular make of weapon used. As a further result of the recoil, the breechblock glides back. At the same time, the recoil spring and the firing pin spring are compressed. According to the particular construction of pistol, the fired shell is ejected either by an ejector stud on the firing pin or by the usual ejector.
The ejector is attached to the frame of the pistol, and ejects the fired shell as the breech block slides rearward. Occasionally, as in the case of the Webley and Scott pistols of caliber 7.65 and 6.35 (with hammer) and the MAB pistol, one of the magazine lips acts as an ejector. The extractor, which catches hold of one side of the shell as the slide glides back, acts as the pivot for the shell as it is being ejected. As a result the extractor hook usually scratches clearly visible marks on the shell head (Fig. 285).
Anyone who wishes to work with shells for the purpose of determining the numerous types of arms used, must necessarily become acquainted with the exact processes and mechanisms of various pistol models, in order to be able to distinguish for himself the several kinds of markings. It will make matters easier if the cartridges to be employed in experimental tests are painted with a mixture of spirit varnish and castor oil. If this is done, the various marks will become more easily visible. During our early studies of breech face impressions on the shell head and primer, we pasted small discs of copper, 0.05 mm. thick (which had been softened by heating in a stream of hydrogen) on the shell head before firing the cartridge. This method is especially valuable in determining whether any of the boundaries of the breech face fall within the area of the shell head, and can be counted on to produce characteristic imprints thereon. After some practice in recognizing breech face impressions, it is well to use cartridges without the head stamps (the letters and numbers commonly imprinted in the course of manufacture), as these make it more difficult to see the markings, and their presence sometimes masks entirely any imprint which may be acquired. Furthermore, cartridges with copper primers are better for our purposes than those with brass ones, because the impressions in the copper are clearer.
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