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In firearms and artillery, the primer (/ˈpraɪmər/) is the chemical and/or device responsible for initiating the propellant combustion that will propel the projectiles out of the gun barrel.
In early black powder guns such as muzzleloaders, the primer was essentially the same chemical as the main propellant (albeit usually in a finer-powdered form), but poured into an external flash pan, where it could be ignited by an ignition source such as a slow match or a flintlock, though some muzzleloaders have primers like cap gun caps. This external powder was connected through a small opening at the rear of the gun barrel that led to the main charge within the barrel. As gunpowder will not burn when wet, this made it difficult (or even impossible) to fire these types of weapons in rainy or humid conditions.
Modern primers, by contrast, are more specialized and distinct from the main propellant they are designed to ignite. They are of two types, those using shock-sensitive chemicals, and those reliant on chemicals ignited by an electric impulse. In smaller weapons the primer is usually of the first type and integrated into the base of a cartridge. Examples include handgun cartridges, rifle cartridges, and shotgun shells. Larger artillery pieces in contrast typically use electric priming. In artillery the primers are frequently a separate component, placed inside the barrel to the rear of the main propellant charge—but there are other examples of guns, including for example some automatic weapons, designed to shoot cartridges with integral electric primers.
Upon being struck with sufficient force generated by the firing pin, or electrically ignited, primers react chemically to produce heat, which gets transferred to the main propellant charge and ignites it, and this, in turn, propels the projectile. Due to their small size, these primers themselves lack the power to shoot the projectile, but still have enough energy to drive a bullet partway into the barrel — a dangerous condition called a squib load.