Which action should be taken immediately after pulling the cocking handle to the rear when unloading the M240?

After pulling the cocking handle to the rear when unloading the M240, the immediate action that should be taken is to place the safety on "S" (safe). This step is crucial for ensuring that the weapon is rendered inoperable while the unloading process is continued. Engaging the safety prevents any accidental discharge during the subsequent steps of the unloading procedure, providing an extra layer of safety for the operator and anyone nearby.

This action serves as a primary safety measure since, even though the weapon is being unloaded, there could still be live rounds in the chamber or the feed mechanism. By placing the weapon on safe immediately after manipulating the cocking handle, the operator minimizes the risk associated with handling the weapon during the unloading process.

While the other options, such as opening the cover, removing ammunition belts and links, and inspecting the chamber, are important parts of the unloading procedure, they follow the critical step of ensuring the weapon is made safe first.