Cartridge (firearms)

A cartridge, also known as a round, is a type of pre-assembled firearm ammunition packaging a projectile (bullet, shot, or slug), a propellant substance (smokeless powder, black powder substitute, 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 convenient transportation and handling during shooting. Although in popular usage the term "bullet" is often used to refer to a complete cartridge, the correct usage only refers 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.

Cartridges can be categorized by the type of primer, which is a small charge of an impact-sensitive explosive compound located:

Only small-caliber rimfire cartridges and centerfire cartridges have survived into the modern day.

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 burning slow match, to a small charge of gunpowder in a flash pan, to a metallic percussion cap mounted on top of a "nipple" or cone), to serve as a source of activation energy 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 the threat of close combat (particularly cavalry charges) as well as complicating the logistics of ammunition.

The primary purpose of using a cartridge is to offer a handy pre-assembled "all-in-one" package that is convenient to handle and transport, easily loaded 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 cartridge case also enables the action mechanism to use part of the propellant's energy (carried inside 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 in a loud blast, and the chamber pressure drops back down to ambient 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.