Basic Firearms Terminology Defined
Accuracy - The ability of a firearm to place all shots close to the same place.
Action - The mechanism of a firearm. The action allows a gun to be loaded, locked, fired, and unloaded.
Ballistic Coefficient (BC) - The ratio of a bullet's sectional density to its coefficient of form. BC represents how efficiently the bullet flies through the air; the higher the BC number the more aerodynamic the bullet.
Brass - The cartridge case. This is usually made out of brass, hence the name. The case holds the powder inside, a primer in its bottom, and a bullet in its mouth. Assembled, these components constitute a cartridge.
Bullet - The projectile that is propelled downrange when a gun is fired. Bullet cores are usually made of lead that is often jacketed or semi-jacketed in copper or a copper alloy. Or not jacketed at all (a.k.a. cast or swaged lead bullets).
Bullet diameter - The actual diameter of a bullet, usually measured in decimal fractions of an inch (.308"). In Europe bullet diameters are typically expressed in millimeters (7.62mm). In recent years many cartridges have been named for their bullet diameter (.308 Winchester).
Bore diameter - The land to land diameter of the inside of a gun barrel, usually expressed in decimal fractions of an inch (.300). Traditionally cartridges have been named for the bore diameter of the guns that fire them (.270 Winchester).
Cartridge - The combination of the bullet, case, powder, and primer in a ready to fire configuration (the way you would buy them in a box from a store).
Case - The case is really a vessel that holds the powder inside, a primer in its bottom, and a bullet in its mouth. It is usually made of brass. Assembled, these components constitute a cartridge.
Firing pin - This is a small metal rod inside of a firearm that strikes the primer of a cartridge when the trigger is pulled and it is hit by the hammer. This causes the primer to ignite the powder inside the cartridge and propel the bullet out of the firearm. A long, heavy firing pin that is propelled by a stiff spring rather than being hit by a hammer is usually called a "striker."
Grains - A unit of measure in which 7000 grains equal one pound and 437.5 grains equal one ounce. Used in the measure of bullet weight and the weight of powder charges, among other things.
Groove diameter - The groove to groove diameter of the inside of a gun barrel. This is usually the same as the diameter of the bullets that it is intended to fire.
Maximum Average Pressure (MAP) - The maximum permissible chamber pressure of a given cartridge as specified by the Small Arms and Ammunition Manufacturers Institute (SAAMI) in the U.S.A. In Europe the similar governing body is CIP.
Primer - This is a small cap that fits into the rear of the brass. This cap usually contains a highly sensitive explosive that is required to set of the powder inside of a cartridge.
SAAMI - The Small Arms and Ammunition Manufacturers Institute, which is the governing body for arms and ammunition standards in the U.S.A.
Sectional Density (SD) - The diameter to weight ratio of a bullet. Calculated by dividing the bullet's weight in pounds by its diameter in inches. Sectional density is important because, all other factors being equal, the higher the SD the better the penetration of the bullet.
Smokeless Powder - The nitro-cellulose based gunpowder that is used in most modern firearms. There are several manufacturers of smokeless powder and numerous individual types of smokeless powder. Among the best known brands in North America are Alliant, Hodgdon, IMR, Norma, and Winchester/Olin.
Striker - A long, relatively heavy firing pin that is propelled by a stiff spring rather than being hit by a hammer.
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