Final answer:
The M2 machine gun has an additional safety mechanism known as the barrel extension assembly safety, besides the primary safety located on the spade grips.
Step-by-step explanation:
The M2 machine gun features multiple safety mechanisms to prevent accidental discharge. Other than the gunner's primary safety, which is usually found on the weapon's spade grips, the M2 machine gun has an additional safety: the barrel extension assembly safety. This safety is located on the barrel extension and prevents the gun from firing unless it is fully locked in place. User training emphasizes the importance of using these safeties correctly to ensure safe operation of the weapon.