What action must the gunner take to change the barrel of the M60E3?

To change the barrel of the M60E3, the gunner must raise the barrel lock lever. This lever is designed to secure the barrel in place during firing operations. By raising it, the gunner releases the lock on the barrel, allowing for safe removal and replacement of the barrel. This action is a critical part of the barrel change procedure particularly after prolonged firing, as barrels can overheat and require changing for continued effective operation. Proper barrel management helps maintain the weapon's reliability and safety in combat situations.

While other actions like removing the ammunition belt or turning the weapon upside down may relate to handling the weapon, they are not the specific steps required for barrel replacement. Releasing the trigger is also not relevant in this context, as it pertains to weapon safety during firing rather than the mechanical operation of changing the barrel.