Which of the following accurately describes the M16 operation?

The M16 rifle is best described as shoulder-fired and air-cooled. It is designed to be fired from the shoulder, making it a more ergonomic choice for soldiers, as it allows for better stability and accuracy during use. The air-cooled aspect refers to the rifle's cooling mechanism, where heat generated during firing dissipates into the air. This enables the weapon to maintain performance over sustained firing without the need for additional cooling systems.

The M16 employs a direct gas impingement system for its operation, where gas from the fired round is used to cycle the action and chamber the next round automatically, which is not accounted for in other descriptions. Additionally, the M16 is capable of both semi-automatic and fully automatic fire modes, making it versatile for different combat situations.