Which component is NOT essential to the M240 machine gun?

The component that is not essential to the M240 machine gun is the scope. While scopes can enhance accuracy and target acquisition, they are not a fundamental part of the weapon's operation. The M240 is designed to function effectively without a scope, relying on open sights for targeting.

In contrast, the barrel, receiver, and operating system are critical to the machine gun’s functionality. The barrel is necessary for directing the projectile, the receiver houses the mechanism and components that allow the weapon to fire, and the operating system is vital for cycling the ammunition and handling the recoil. These components are integral to the weapon's overall design and performance, making them essential for its operation.