What is the definition of "Firing" in the cycle of operations of the M16A2?

The definition of "Firing" within the cycle of operations of the M16A2 is specifically engaging the trigger. This action is critical as it initiates the process that results in the discharge of a round. When the trigger is pulled, it releases the hammer, which strikes the firing pin, causing it to impact the primer of the cartridge. This ignition process ultimately leads to the firing of the bullet from the chamber.

While sending gas back down the barrel, releasing the bolt, or loading a new round are all important components of the weapon's operation, they pertain to different stages of the firing cycle rather than the act of firing itself. Engaging the trigger is the definitive action that commences the firing sequence, making it the correct answer.