Which of the following does NOT refer to a part of a bullet or cartridge?

The term "trigger" does not refer to a part of a bullet or cartridge. Instead, the trigger is a component of a firearm itself, specifically a part of the mechanism that initiates the firing process when pulled.

In contrast, the bullet, primer, and casing are all integral components of a cartridge. The bullet is the projectile that is expelled from the firearm upon firing. The primer is a small explosive device that ignites the gunpowder when struck by the firing pin, causing the bullet to be propelled forward. The casing is the protective shell that holds the primer, gunpowder, and bullet together in a fully assembled cartridge. Understanding these distinctions is key in firearms terminology and operation.