What command must be given before double-timing with an M16?

Before double-timing with an M16, the command given is "Port Arms" or "Sling Arms." This command is essential because it ensures that the rifle is positioned safely and securely while in motion. When a soldier is double-timing, it is important to maintain control of the weapon to prevent accidents and to ensure the soldier can maneuver effectively.

"Port Arms" positions the rifle at a safe angle where it can be readily handled, while "Sling Arms" allows the weapon to be secured to the body, providing hands-free mobility. This readiness is crucial in military operations where both speed and weapon control are necessary.

The other commands such as "Load arms," "Present Arms," and "Attention" do not relate to the specific context of preparing to double-time with an M16, as they serve different purposes in drill and ceremony or in combat readiness scenarios.