What is known as a round in the context of firearms?

In the context of firearms, a "round" refers specifically to a complete unit of ammunition used in firearms, which comprises all components of a single cartridge. This includes the bullet (the projectile), case (which holds all parts together), primer (that initiates the firing process), and gunpowder (the propellant). Understanding this definition is crucial because it encapsulates everything needed for the ammunition to function properly when loaded into a weapon.

The other options describe different aspects of ammunition or related components but do not encompass the complete concept of what constitutes a "round." For example, a measurement for bullets may refer to various specifications such as caliber, but it does not reflect the entire cartridge. Similarly, a type of shotgun shell is a specific kind of round used in shotguns, and the propelling agent is merely one part of a cartridge and not the complete assembly. Thus, option A provides the most comprehensive and accurate definition of a round in the context of firearms.