Cartridge case design is a science in itself. There are certain basic requirements - the case must locate the projectile in precisely the correct position in the firing chamber (with the exception of advanced primer ignition - API - blowback guns which fire as the case is moving forwards); it must be easy to extract once fired; and it must function well in automatic weapons - but there are various methods of achieving these aims.
Before proceeding further it is well to be clear about the nomenclature of cartridge cases. The basic elements - head, rim and neck - are shown in the drawing on page 10 (unfortunately these terms are rather confusing in that the head is as far from the neck as possible!). Cases usually have an identifying headstamp around the primer pocket, which may contain information about the cartridge, the manufacturer and the year of production. Rimless cases need an extractor groove and 'bottleneck* cases also have a shoulder, where the case is reduced in diameter down to the neck in cartridges in which the calibre is significantly smaller than the case diameter. Some cases have a belt - an annular projection -just above the extractor groove.
In modern small-arms cartridges the inside neck diameter is made fractionally smaller than the bullet, thereby ensuring that the bullet is gripped firmly until it is fired. In the larger calibres considered here, a more authoritative means of securing the projectile is normally required and this is provided by crimping: the case neck is pressed into recesses in the projectile, either all around the circumference or at several points on it.
Accurate location in the firing chamber is essential to the ignition of the primer; the cartridge needs to enter the chamber, but not too far or the firing pin (or electrical contact) will not touch the primer. It is also important in the internal ballistics of the gun, as the projectile must be in the same place, relative to the conical section leading to 'the rifling, to achieve consistent pressure characteristics. The distance between the face of the bolt and the part of the chamber which locates the cartridge is known as the kheadspace\
The earliest method of achieving accurate location was by means of a rim around the head of the case. This remains outside the chamber or in a recess so that it is flush with the breech face. The rim also provides something for the extractor to hook on to in order to pull the fired case from the chamber. Rimmed cases are still satisfactory except that the rim can complicate ammunition-feed arrangements in automatic weapons. In spring-loaded magazines the rims can foul each other if improperly loaded, while in belt-fed guns it is usually necessary for the cartridge to be withdrawn backwards from the belt before being pushed forwards into the chamber. This design has therefore largely been replaced for military purposes, although it is still encountered, for example, in some Russian small arms and in larger cannon from 40mm upwards.
By far the most common military case type is the rimless, in which the rim is reduced to the same diameter as the case so that the cartridges in magazines can be stacked on top of each other or pushed forward from ammunition belts without risk of jamming. To give the extractor something to hook on to, an extractor groove is cut into the case. There have been a few examples of semi-rimmed (or semi-rimless) cases, in which the rim is only fractionally wider than the case and is combined with a small
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