In firearms terminology, what defines a 'round'?

A 'round' in firearms terminology refers to a complete ammunition cartridge, which consists of several key components: the bullet (projectile), gunpowder (propellant), casing (which holds everything together), and primer (which ignites the gunpowder). The completeness of all these elements is what distinguishes a 'round' from an empty cartridge casing, which lacks the necessary components to function as ammunition.

Understanding the composition of a round is essential for discussions around ammunition types, usage, and safety in handling firearms. A complete ammunition cartridge is the essential item needed for firing a projectile from a firearm, making this definition vital in both practical use and theoretical discussions in firearms training and safety protocols.

The other options relate to important concepts in ammunition, like the casing or specific types of cartridges, but they do not encompass the full definition of what constitutes a 'round.'