Knowing how ammunition works is essential to getting the most out of your firearm. Learn about the core components of ammunition, basic terms and standard measurements.

Components of ammunition


(1) Bullet: The metal projectile (usually lead) that is expelled from the mouth of a firing cartridge.

(2) Casing: A tube that houses the cartridge primer and powder, and is typically made from brass. The bullet is typically held in the open end of the casing.

(3) Powder: The gunpowder used in a cartridge that propels the bullet.

(4) Primer: A small cap in the head of the cartridge that when struck by the firing pin, ignites the powder.

How does firing ammunition work?

When the gun is fired, the primer is hit by the firing pin of the gun and ignites the powder. As the powder burns, it creates pressure that pushes the bullet (projectile) down the barrel.

How is ammunition measured?

Ammunition is measured by the bullet's diameter. For example, a 9mm bullet is approximately 9.01mm and a .40 caliber bullet is .400 inches in diameter. Depending on the origin of its invention, ammunition is measured using millimeters or caliber. 9mm ammunition originates in Europe, whereas the .40 caliber was invented in the United States.

Basic Ammunition Calibers

(1) 9mm: This caliber is the most popular and widely used worldwide.

(2) .40 caliber: Larger and slower than 9mm, with more recoil.

(3) .45 auto: Slightly larger and slower than .40 caliber, with a bit more recoil.

(4) .45 gap: Rivals the firepower of the .45 auto, but is shorter, allowing it to fit into a more compact semi-auto pistol.

(5) 10mm: Has greater range and is faster than the .45 auto.

(6) .357 auto: Slightly longer than the .40 caliber, and known for accuracy and stopping power.

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