Modem styles In case manufacture.
greatly increase the strength and safety of our present rifles, but is unlikely that this generation will sec any steel eases used for commcrcial ammunition, unless some cheaper method of manufacture than we have at present is devised.
A detailed description of cartridge case manufacture would be of little practical use to the reloadcr but some understanding of the process and of the physical nature of the finished case is necessary in order to understand the changes that take place when the case is subjected to the strain of firing.
The general process consists in blanking out discs from strip brass and forming the discs into cups. These cups are forced or drawn through successive dies which elongate them, at the same time reducing their thickness and diameter. The heads and primer pockets are formed by cold forging the bases of the cups and the rims or extractor grooves are turned. The cases are given the proper taper or botdc-neck form by forcing one or more dies of the 7 proper shape over them, after which they axe trimmed to the correct length.
The severe stresses and strains that cartridge cases are subjected to when fired, makes it ncccssary to use only brass of the highest quality in their manufacture for, it must be remembered, the cartridge case is part of the arm it is fired in and the safety of the arm and the shooter depend largely upon the strength of the case. The usual alloy used is about 70% of copper to 30% of spelter (zinc) and every care is taken to exclude impurities and other metals. In foundry parlance, "cartridge brass," means just about the finest brass that it is possible to procure.
Brass, when etched and viewed under a microscope, appears as a crystalline structure. The size and form of the crystals depends upon two things; cold work and annealing. Working, that is, drawing, bending or compressing brass while it is cold, hardens it by stretching or compressing the crystals, while annealing softens it, causing the crystals to re-form. The greater the heat and the longer its duration, the larger the crystals becomc.
In manufacturing cases, it is necessary to anneal the cups between each drawing operation and also before they are tapered or finish-formed. These anneals are carefully worked out with relation to the amount of cold work to be performed after them, so that the finished case will have the proper degree of hardness. The case must not be too hard or it will rupture when fired. On the other hand, if it is too soft it may give way when fired, ruining the rifle and possibly the shooter as well. All parts of the case are not of the same degree of hardness. In general, the head is of a tough, coarse structure, with the side walls gradually increasing in hardness towards the mouth of the case. The physical characteristics of different calibers are not the same, as each one presents its own metallurgical problems. However, what we are interested in knowing here is, that a cartridge case has a crystalline formation, that it is carefully
8 made to give it the proper strength and that both of these factors can be changed when the cartridge case is fired, although they normally are not.
As the cartridge case is a part of the arm it is fired in, it is ncccssary to understand and to give some consideration to the arm, in order to understand the changes that take place in the case when a cartridge is fired. This change may be, and normally is, negligible, which has given rise to the general statement that cartridge cases are just as good and serviceable after firing as they were before. This statement is substantially correct, but not literally so, because some change docs take place during the firing. It is perfecdy obvious that if no change took place the first time the case were fired there would be no change the second time and so on ad infinitum. The kind and amount of change depends principally upon four things; the- relation of the size and shape of the case to the chamber it is fired in, the pressure developed within it, the thickness and temper or hardness of the brass itself, and the products of combustion that are left in the case after firing. Any of these things or any combination of them can, under some circumstances, render a cartridge unsafe for reloading . . . but they usually don't.
Relation of Cartridge to Chamber. The chambcr of an arm is the recess provided in the rear of the barrel or cylinder to receive the cartridge. As cartridge cases are made of springy brass, it is impossible to make them all exactly alike and the cases of each caliber will, if measured very carefully at all points, be found to differ slightly. This variation in dimensions will not only be found in different makes of cartridges of the same caliber but are present in cartridges of the same make and even among those from one lot produced on the same machines. The uniformity in dimensions of all calibers of our American ammunition is truly remarkable and the little differences referred to here are what are known as manufacturing tolerances^ or the
9 slight differences that can be permitted without affecting the serviceability of the finished product. The smallest cartridge of any given caliber is known as a minimum cartridge, while the largest permissible cartridge is called a maximum cartridge. The differences in any of the dimcn-
Was this article helpful?