Hold command-option and click 'New Game' for level select.
On the first level, go to the first room out of the two at the end of the corridor, then kill the android and the Phors. Go to the top left-hand corner and push action. Ride the lift up and blast the Phors. Go through the door and Whammo! You see a Combat Shotgun. Get that, go out, then come back. More ammo appears! That only works once.
