A cartridge or a round is a type of pre-assembled firearm ammunition packaging a projectile (bullet, shot, or slug), a propellant substance (usually either smokeless powder or black powder) and an ignition device (primer) within a metallic, paper, or plastic case that is precisely made to fit within the barrel chamber of a breechloading gun, for the practical purpose of convenient transportation and handling during shooting. Although in popular usage the term "bullet" is often informally used to refer to a complete cartridge, it is correctly used only to refer to the projectile.
Military and commercial producers continue to pursue the goal of caseless ammunition. Some artillery ammunition uses the same cartridge concept as found in small arms. In other cases, the artillery shell is separate from the propellant charge.
A cartridge without a projectile is called a blank; one that is completely inert (contains no active primer and no propellant) is called a dummy; one that failed to ignite and shoot off the projectile is called a dud; and one that ignited but failed to sufficiently push the projectile out of the barrel is called a squib.
The cartridge was invented specifically for breechloading firearms. Prior to its invention, the projectiles and propellant were carried separately and had to be individually loaded via the muzzle into the gun barrel before firing, then have a separate ignitor compound (from a slow match, a small charge of gunpowder in a flash pan, or a percussion cap) to set off the shot. Such loading procedures often require adding paper/cloth wadding and ramming down repeatedly with a rod to optimize the gas seal, and are thus clumsy and inconvenient, severely restricting the practical rate of fire of the weapon, leaving the shooter vulnerable to close combat (particularly cavalry charges) as well as complicating the logistics of ammunition.
The primary purpose of a cartridge is to offer a handy pre-assembled "all-in-one" package that is convenient to handle and transport, easily inserted into the breech (rear end) of the barrel, as well as preventing potential propellant loss, contamination or degradation from moisture and the elements. In modern self-loading firearms, the round also enables the action mechanism to use part of the propellant energy (carried by the cartridge itself) and cyclically load new rounds of ammunition to allow quick repeated firing.
To perform a firing, the round is first inserted into a "ready" position within the chamber aligned with the bore axis (i.e. "in battery"). While in the chamber, the cartridge case obturates all other directions except the bore to the front, reinforced by a breechblock or a locked bolt from behind, designating the forward direction as the path of least resistance. When the trigger is pulled, the sear disengages and releases the hammer/striker, causing the firing pin to impact the primer embedded in the base of the cartridge. The shock-sensitive chemical in the primer then creates a jet of sparks that travels into the case and ignites the main propellant charge within, causing the powders to deflagrate (but not detonate). This rapid exothermic combustion yields a mixture of highly energetic gases and generates a very high pressure inside the case, often fire-forming it against the chamber wall. When the pressure builds up sufficiently to overcome the fastening friction between the projectile (e.g. bullet) and the case neck, the projectile will detach from the case and, pushed by the expanding high-pressure gases behind it, move down the bore and out the muzzle at extremely high speed. After the bullet exits the barrel, the gases are released to the surroundings as ejectae, and the chamber pressure drops back down to atmospheric level. The case, which had been elastically expanded by high pressure, contracts slightly, which eases its removal from the chamber when pulled by the extractor. The spent cartridge, with its projectile and propellant gone but the case still containing a used-up primer, then gets ejected from the gun to clear room for a subsequent new round.
While historically paper had been used in the earliest cartridges, almost all modern cartridges use metallic casing. The modern metallic case can either be a "bottleneck" one, whose frontal portion near the end opening (known as the "case neck") has a noticeably smaller diameter than the main part of the case ("case body"), with a noticeably angled slope ("case shoulder") in between, or a "straight-walled" one, where there is no narrowed neck and the whole case looks cylindrical. The case shape is meant to match exactly to the chamber of the gun that fires it, and the "neck", "shoulder", and "body" of a bottleneck cartridge have corresponding counterparts in the chamber known as the "chamber neck", "chamber shoulder", and "chamber body". Some cartridges, like the .470 Capstick, have what is known as a "ghost shoulder" which has a very slightly protruding shoulder, and can be viewed as a something between a bottleneck and straight-walled case. A ghost shoulder, rather than a continuous taper on the case wall, helps the cartridge to line up concentrically with the bore axis, contributing to accuracy. The front opening of the case neck, which receives and fastens the bullet via crimping, is known as the case mouth. The closed-off rear end of the case body, which holds the primer and technically is the case base, is ironically called the case head as it is the most prominent and frequently the widest part of the case. There is a circumferential flange at the case head called a rim, which provides a lip for the extractor to engage. Depending on whether and how the rim protrudes beyond the maximum case body diameter, the case can be classified as either "rimmed", "semi-rimmed", "rimless", "rebated", or "belted".
The shape of a bottleneck cartridge case (e.g. body diameter, shoulder slant angle and position, neck length) also affects the amount of attainable pressure inside the case, which in turn influences the accelerative capacity of the projectile. Wildcat cartridges are often made by reshaping the case of an existing cartridge.
In addition to case shape, rifle cartridges can also be grouped according to the case dimensions, which in turn dictates the minimal receiver size and operating space (bolt travel) needed by the action, into either "mini-action", "short-action", "long-action" ("standard-action"), or "magnum" categories.
The most popular material used to make cartridge cases is brass due to its good corrosion resistance. The head of a brass case can be work-hardened to withstand the high pressures, and allow for manipulation via extraction and ejection without rupturing. The neck and body portion of a brass case is easily annealed to make the case ductile enough to allow reshaping so that it can be handloaded many times, and fire forming can help accurize the shooting.
Steel casing is used in some plinking ammunition, as well as in some military ammunition (mainly from the former Soviet Union and China). Steel is less expensive to make than brass, but it is far less corrosion-resistant and not feasible to reuse and reload. Military forces typically consider service small arms cartridge cases to be disposable, single-use devices. However, the mass of the cartridges can affect how much ammunition a soldier can carry, so the lighter steel cases do have a logistic advantage. Conversely, steel is more susceptible to contamination and damage so all such cases are varnished or otherwise sealed against the elements. One downside caused by the increased strength of steel in the neck of these cases (compared to the annealed neck of a brass case) is that propellant gas can blow back past the neck and leak into the chamber. Constituents of these gases condense on the (relatively cold) chamber wall, and this solid propellant residue can make extraction of fired cases difficult. This is less of a problem for small arms of the former Warsaw Pact nations, which were designed with much looser chamber tolerances than NATO weapons.
Aluminum-cased cartridges are available commercially. These are generally not reloaded, as aluminum fatigues easily during firing and resizing. Some calibers also have non-standard primer sizes to discourage reloaders from attempting to reuse these cases.
As firearms are projectile weapons, the projectile is the effector component of the cartridge, and is actually responsible for reaching, impacting, and exerting damage onto a target. The word "projectile" is an umbrella term that describes any type of kinetic object launched into ballistic flight, but due to the ubiquity of rifled firearms shooting bullets, the term has become somewhat a technical synonym for bullets among the handloading crowds. The projectile's motion in flight is known as its external ballistics, and its behavior upon impacting an object is known as its terminal ballistics.
The oldest gun propellant was black gunpowder, a low explosive made from a mixture of sulfur, carbon and potassium nitrate, invented in China during the 9th century as one of Four Great Inventions, and still remains in occasional use as a solid propellant (mostly for antique firearms and pyrotechnics). Modern firearm propellants however are smokeless powders based on nitrocellulose and nitroglycerin, first invented during the late 19th century as a cleaner and better-performing replacement for black powder. Modern smokeless powder may be corned into small spherical balls, or extruded into cylinders or strips with many cross-sectional shapes using solvents such as ether, which can be cut into short ("flakes") or long pieces ("cords").
Modern primers are basically improved percussion caps with shock-sensitive chemicals (e.g. lead styphnate) enclosed in a small button-shaped capsule. In the early paper cartridges, invented not long after the percussion cap, the primer was located deep inside the cartridge just behind the bullet, requiring a very thin and elongated firing pin to pierce the paper casing. Such guns were known as needle guns, the most famous of which was decisive in the Prussian victory over the Austrians at Königgrätz in 1866. After the metallic cartridge was invented, the primer was relocated backward to the base of the case, either at the center of the case head (centerfire), inside the rim (rimfire), inside a cup-like concavity of the case base (cupfire), in a pin-shaped sideways projection (pinfire), in a lip-like flange (lipfire), or in a small nipple-like bulge at the case base (teat-fire). Today, only the centerfire and rimfire have survived the test of time as the mainstream primer designs, while the pinfire also still exists but only in rare novelty miniature guns and a few very small blank cartridges designed as noisemakers. 041b061a72