A and B are the caps of two different -303 cartridges, both fired by the same rifle. The right hand photograph is composite, the central portion of B being superimposed over the corresponding portion of A. The exact matching of all the striations on the two different caps caused by the imprint of the breech face of the bolt is convincing and obvious. There could be no possible doubt that both these cartridges must have been fired by the same rifle movement of the pin as well as longitudinal movement. While if the pin is itself not quite straight the direction of the blow which it delivers may vary to an appreciable extent from round to round.
And if the direction of the blow is not perfectly straight the "ex-centricity" of the striker indentation may appear to be quite different in a high-pressure round from what it appears to be in a low-pressure round. This is because the indentation is much deeper in a high-pressure round, and so if the blow is slanting the divergence from the centre of the cartridge of the bottom of the indentation will vary with the depth of the indentation.
All the same, the "ex-centricity" of the striker indentation can be a very useful bit of subsidiary evidence even in cartridges fired from self-loading pistols provided its limitations are appreciated.
I have just stated that the depth of the striker indentation is dependent on the pressure. In fact the appearance of the striker indentation is one of the first points for which a trained ballistician would look when assessing the degree of pressure developed in a fired cartridge. Yet it has been asserted that the measurement of die depth of the striker indentation is one of the methods of determining whether two fired cartridges have both been fired by the same weapon. The best refutation of the entirely erroneous idea that a particular weapon always delivers a striker indentation of constant depth is Plate XXIII, which shows two photographs of sections of cartridges both of which were fired by the same self-loading pistol. The striker indentation is clearly shown in profile in these photographs and the difference in depth is too obvious to need words.
As a matter of fact the depth is also dependent on the thickness of the rim in the case of revolver cartridges, and in the gauge and taper of the cases of self-loading pistol cartridges.
But the important point to realise is that the depth of the striker indentation is useless as a means of identifying firearms.
In '22 rifles, which are universally used in "Small Bore Shooting," the cartridge is always a rim-fire, as shown Plate VI (A), Fig. I, and the striker hits the base of the cartridge near the rim and in prolongation with the wall of the case. These rim-fire striker indentations can vary considerably in different makes of weapon, both in shape and position. Six different types of these -22 rim fire striker indentations are shown in Plate II (b) and (c).
Probably no single class of weapon can vary more in the shape of its strikers than rifles and pistols of '22 calibre. The cartridge is a rim-fire cartridge and has no central cap, the whole of the base containing a thin layer of the explosive composition. As can be understood from the generic name, the striker does not hit the base of the cartridge in the centre, but on the outside edge or rim. The blow which is thus delivered compresses the cap composition between the nose of the striker and the wall of the cartridge case, and it is the wall of the case which takes the place of an anvil. It will, therefore, be obvious that in order to ensure maximum efficiency and certainty of ignition the centre of the striker blow should be delivered exactly over the line of the wall of the cartridge case. In some weapons this does not always occur, and uncertain results as well as mis-fires can then be expected.
Plate II (b) and (c) show six typical but wholly different -22 striker indentations and emphasises how great the differences may be both in the actual size and shape of the indentation as well as in its position.
Sometimes there occurs in a striker indentation what is known as a "striker scrape." An example of such a mark is shown in Plate XXIII. This scrape is usually caused by slighdy too long a striker. When the cartridge case is turned off the breech face on extraction there may not be sufficient clearance in the striker indentation for an abnormally long striker, and consequently the end of the striker scrapes the side of the indentation in the cap.
When a striker scrape is noted it can be of great help as it enables one to orientate the fired case correctly, that is to place it in the same position which it must have occupied in the chamber. It is always most useful to know, and frequendy difficult to find out, how the cartridge fitted in the chamber, that is which way it was up. But a striker scrape enables one to settle this question at once.
It should be remembered that the orientation of a case by means of a striker scrape must depend on the type of weapon in which the cartridge was fired. For example, in an ordinary shotgun with a break-down action the striker scrape must always be at 6 o'clock. But in a self-loading pistol, or indeed any bolt-action weapon, in which the fired case is lifted away from the breech face in a different manner the striker scrape will correspond exactly with the ejector mark. That is if the ejector mark is at 8 o'clock the striker scrape will also be at 8 o'clock.
Sometimes a striker scrape is caused by an abnormally high pressure which so forces the cap back that it moulds itself round the striker which scrapes one side of the cap indentation when the fired cartridge is lifted off the breech face,
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