Modern cartridges have a case which holds bullet, propellant and primer in a single self-contained system. This complete unit and the bullet fired from it are also referred to as a round or shell.
The cartridge is perhaps the single most important factor making modern firearms practical. The cartridge is a stand-alone module of mechanical integrity sealing sensitive chemical compounds from the external environment, and keeping together all the consumable components needed for firing one round. This key development enabled the significant advance of arms technology starting in the latter half of the 19th century.
One of the topics not covered in this document is ballistics. Internal ballistics is the study of bullet and barrel performance while the bullet is in the barrel. External ballistics studies the bullet in free flight from muzzle to target. Terminal ballistics is the study of the effects of the projectile in the target medium. Targets studied usually include armor plating, sheet metal, flesh, or flesh simulants such as ballistic gelatin. See the Bibliography for some ballistics references.
It was the .50 caliber Browning rifle bullet which helped shape the first successful manned supersonic aircraft (Chuck Yaeger's Bell X-1) since it was one of the few objects known to be aerodynamically stable at three times the speed of sound.
The most important role of the case after feeding is the sealing of the breech. During firing the case expands outward against the barrel chamber providing a gas-tight seal and preventing the high pressure gases from entering the rest of the arm. Such a release could case a failure hazardous to the user, since usually only the combined case/chamber/breech system has sufficient strength to withstand the high pressures generated. Since the case expands under great pressure against the chamber, considerable force is required to overcome friction in extracting the fired case from the chamber.
In modern rifle and pistol rounds the primer is press fit and sometimes crimped into the back of the cartridge, where the striker can reach it. Cannon rounds are usually electrically fired. That is, the primer is an electrical device with two insulated, usually concentric terminals which is ignited through electrical current. In rimfire rounds, the primer compound is in the fold at the back of case which also forms the extraction rim. The striker impacts the rim, detonating the primer material.
Modern powders are extruded in the shape of rods or discs and come in a variety of sizes which together with chemical composition affect the burning rate. Smaller-grained, fast-burning propellants are generally used in higher velocity applications and larger-grained, slower-burning used in lower velocity rounds. Matching of burning rates to bore diameters and projectile masses requires careful calculation and measurement of pressure versus time. This is important to keep within the safe pressure limits of the barrel and rest of the system. Such development can be aided by using piezoelectric cells or strain gauges feeding computerized data acquisition systems. Once calibration procedures are established, these measurement systems are more efficient and simpler to use than old-style mechanical copper crushers.
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