Priming compositions for centerfire ammunition are housed in small metal cups which fit into a recess, called the primer pocket, in the center of the base of the cartridge case. In rimfire ammunition the priming composition is housed inside the cartridge case in the hollow perimeter of the base.
The ideal primer cup metal should expand easily to provide a gas-tight seal, be strong enough to withstand the blow from the firing pin, and also strong enough to withstand the "explosion" of the priming composition and the discharge gas pressure, even though it has been severely dented by the firing pin.
Primer cups are usually made of cartridge brass, although copper, nickel-plated copper or brass, copper alloy, cupronickel, and zinc-coated steel cups are also encountered. Primer cups for use with black powder were usually made of soft copper because of the weaker firing pin blow experienced with old black powder firearms, and the much lower pressures generated by black powder discharge. On the other hand, smokeless powders typically give much higher pressures than black powder and are much harder to ignite. Smokeless powders require a much "hotter" primer, which needs a much stronger blow from the firing pin. Consequently soft copper cups are suitable only for use with low-pressure ammunition.
Two specifications for primer cup metal are (a) 95% to 98% copper and 5% to 2% zinc with not more than 0.05% lead, 0.1% arsenic, 0.002% bismuth, 0.01% antimony, and no more than a trace of any other impurity43; (b) 72% to 74% copper and 28% to 26% zinc with the total impurities not exceeding 0.1% and not more than 0.1% lead and 0.05% iron.44
There are two types of primers used in centerfire ammunition which differ only in physical design. In European countries, the Berdan primer design is preferred, whereas in Canada and the United States the Boxer primer design is favored. The only difference between the two types is the design: the Berdan primer does not have an integral anvil, as the anvil is part of the cartridge case, whereas the Boxer primer has its own anvil which is inserted into the primer cup. Boxer primers are preferred because they are replaceable. Figure 8.1 shows Berdan and Boxer primers.
The Berdan cup is varnished internally when empty and after filling it is covered with a paper disc and then sealed with varnish.
Berdan Primer Boxer Primer
Berdan Primer Boxer Primer
Cupronickel and copper alloy cups that are filled with a mercury fulminate-based primer composition are closed with a tinfoil disc that is varnished on the side that is in contact with the primer composition. A varnish that is frequently used for this purpose is shellac grade 1. After fitting, the cup annulus is coated with a clear varnish to prevent the ingress of moisture or oil.
Generally speaking primer cups for rifles differ in size, structure, and amount of priming composition from those used for pistols and revolvers. Primer cups for use in rifles, pistols, and revolvers range in size from 0.175 to 0.210 inches in diameter. For shotgun cartridges, the primer cup is typically in the range 0.240 to 0.245 inches in diameter.
Although pistol and revolver primer cups may have the same diameter as rifle primer cups, rifle primer cups have a greater cup metal thickness and contain larger amounts of priming composition which is accommodated by a longer primer cup length. The increased thickness of rifle primer cups is necessary because of the heavier blow they receive from the firing pin and the higher working pressures experienced. The larger amount of priming composition is necessary because of the larger amount of propellant used in rifle ammunition.
The weight of primer composition for pistol, revolver, rifle, and shotgun ammunition can range from as little as 0.013 g to as much as 0.352 g depending on the caliber and type of ammunition, but is typically in the region of 0.05 to 0.12 g.
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