What defines a machine gun according to regulations?

A machine gun is specifically defined by its ability to automatically fire multiple rounds with a single pull of the trigger; therefore, the correct choice accurately reflects this definition. This characteristic distinguishes machine guns from other firearms that require separate trigger pulls for each shot.

The key element of a machine gun is its capability to fire continuously or in bursts, which is tied to its design to deliver a high rate of fire without needing the shooter to release and pull the trigger repeatedly. Understanding this fundamental aspect of machine guns is crucial in both legal contexts and practical applications, as it affects how these weapons are categorized and regulated.

In contrast, options that suggest a manually operated system or those requiring multiple trigger pulls do not align with the defining attributes of a machine gun, as they do not provide the same automatic firing capability. Thus, recognizing the unique functionality of a machine gun is essential for comprehending the broader regulatory framework that governs firearms.