Instructions
Play through the game until you reach "Overtime" mode. This mode occurs after the initial 72 hours on the timer are up and the mall has been overrun with mercenaries.
Kill one of the mercenaries that are in the mall. Pick up the machine gun that he drops after he has been defeated. Equip the machine gun.
Hold the "Right" trigger button down to enter "Aim View" mode. Use the left joystick to aim. Press the "X" button to fire the machine gun. The machine gun has 150 rounds and you have to hit either a zombie or a person with each round. When you are successful you'll get the "Perfect Gunner" achievement, and the Ammo Belt will be available at your hideout. It will be in one of the paper bags on the opposite side of the duct entrance.