The cartridge is a marvel of engineering simplicity. A metal case, loaded with propellant ahead of an ignition device and topped with a projectile. Case, primer, powder and bullet. Together they form one round of fixed ammunition. Fixed because the bullet is crimped in the case and is not loaded as a separate component.
Bullet Primers; Boxer & Berdan
Primers come in two separate species, the Boxer is a primer that contains the anvil inside the primer. When fired the firing pin crushes the primer shell compacting the primer mixture between the shell and anvil, causing it to ignite. The primer flash sparks into the powder igniting the charge. Boxer, Colonel Edward Mounier Boxer of the Royal Arsenal introduced his primer design about 1866. The Boxer primer is the one popular in the US and facilitates reloading. The other, popular in European military, is the Berdan primer. Invented by an American, Hiram Berdan. The only differences are that the Berdan primer’s anvil is in the case, not self-contained as in the Boxer. The Berdan’s have two flash holes instead of one centrally located like the Boxer. Most European military ammo is thus very difficult to reload.
Primer Dimensions & Sizes
Rifle primers are .008” taller than pistol primers. Diameters are SR- small rifle primers at 0.175” (4.45mm) as are SP-small pistol. Their large rifle (LR) and large pistol (LP) cousins are 0.210” (5.33mm). Shotguns all use the 0.209” (5.31mm) #209 primers. Those into long range precision competition shooting the .50 caliber BMG use 0.315” (8.00mm) primers.
Powder is a nitrocellulose based propellant and is referred to as single base powder. Double base powder adds a pinch of nitroglycerin for spice. Powder grains come in spherical, flake and tubular forms. The tubular forms are progressive burn, as they have a perforation running their length kind of like a mini-pipe. This allows the powder to burn from the inside out as well as the outside in. This maintains a constant burn rate. All other powder grains burn from the outside in, with a diminishing surface area with more gas produced early and progressively less throughout the combustion process.
The case, sometimes made of mild steel with Berdan primers and either a lacquer coating a copper wash (very thin coating of copper) while high quality military and sporting ammo usually have brass cases (a copper-zinc alloy). Those with a flange or flat rim larger in diameter than the body are referred to as rimmed. Lever action, single and double barreled dangerous game and revolver cartridges are usually rimmed. A cartridge head with only an extraction grove and the same diameter as the base is a rimless and common in auto pistol and most rifles. The belted case has a belt around the base and head that is larger than the case and usually identifies the case as a magnum. Rebated rims, rim smaller than the case follows. Rebated cartridges are not very common. Lastly the cases are described as straight wall with nearly parallel sides and bottle necked with a large diameter body tapering down to hold a bullet.
Purposes of Cartridge Case
Cases are not only used to contain all the components needed to constitute a round of ammo, but they serve two more functions. When fired the case expands sealing the chamber and preventing the gas from flashing back towards the shooter, this is referred to as obturation. When extracted the cartridge removes heat, acting as a cooling mechanism, important in semi-automatic or automatic fire, i.e. machineguns. For such a relatively simple device the modern cartridge performs multiple functions.