What is a machine gun?

A machine gun is specifically defined as a firearm that is capable of sustained automatic fire, utilizing bullets fed through a mechanical loading system. This characteristic makes it distinct from other types of firearms, which may fire single shots or utilize different mechanisms for ammunition delivery.

Machine guns typically operate by using the energy from the fired round or a recoil mechanism to cycle the action and load the next round, allowing for rapid and continuous fire as long as the trigger is held down. This operational feature is critical in military applications, providing significant firepower in combat situations.

The other choices describe different types of weapons or functions that do not align with the definition of a machine gun. A shell-firing weapon refers to artillery or mortar systems, while a type of grenade launcher is a distinct category designed specifically for launching grenades. Lastly, a weapon that fires non-lethal rounds is designed for use in less-lethal scenarios, typically for crowd control, and does not have the same lethality or function as a traditional machine gun.