What is the additional step required for performing a function check on the M240?

The additional step required for performing a function check on the M240 is to place the safety on "S" (Safety). This step is crucial because it ensures that the weapon is in a safe condition before any checks are performed. By engaging the safety, you prevent accidental firing during the function check, which typically involves manipulating the operating system of the weapon, such as cycling the action and checking for proper function without the risk of live ammunition being discharged.

The safety mechanism is designed to block the firing mechanism, which adds a layer of safety for the operator. Given that function checks can involve actions where fingers may inadvertently touch the trigger, ensuring the safety is on mitigates that risk. Thus, placing the safety on "S" is a fundamental procedure that must be followed to uphold safe weapon handling practices in military and operational environments.