Which of the following is NOT one of the major components of the M16A3/M4A1 service rifle?

The correct choice identifies the stock assembly as not being categorized as one of the major components of the M16A3/M4A1 service rifle. In the context of the rifle, the major components are typically defined as those that are essential for the rifle's operation and function.

The upper receiver assembly includes the barrel, bolt carrier group, and other critical components involved in the firing and cycling of the rifle. Similarly, the lower receiver assembly houses the trigger mechanism, magazine well, and other essential controls that directly relate to the operational capacity of the rifle. The magazine group assembly, while a distinct part that allows for ammunition feeding, is also crucial in the rifle's functioning.

In contrast, the stock assembly, while important for the overall structure and usability of the rifle, does not directly contribute to the firing mechanism or the essential operation of weapon systems. This distinguishes it from the other components, which are fundamental to the weapon's ability to fire and function effectively.