Which of the following best describes the M16A2 service rifle?

The characterization of the M16A2 service rifle as a 5.56mm, lightweight, magazine-fed, gas-operated weapon is accurate and highlights its key features effectively.

The M16A2 is designed for combat efficiency, incorporating a lightweight construction that enables ease of carry and operation over extended periods. The designation of the caliber as 5.56mm is also significant, as this is the standard NATO caliber, which contributes to its versatility and compatibility with various ammunition types used by allied forces.

Being magazine-fed allows for quick reloading and sustained fire capacity, making it advantageous in combat scenarios. The gas-operated mechanism is a crucial aspect of its operation, utilizing the gas from fired rounds to cycle the action automatically. This emphasizes the rifle's semi-automatic function, enabling a rapid follow-up shot capability without the need for manual cycling of the action.

In contrast, the other descriptions do not align with the actual specifications or operational qualities of the M16A2, making them inaccurate representations. The focus on the correct features in the chosen answer illustrates a clear understanding of the rifle's role and functionality within the U.S. Marine Corps.