To unlock Full Weapon Mode, score a 100 rank in One Weapon Mode. Full Weapon Mode is the exact opposite of One Weapon Mode.

To unlock One Weapon Mode, you must complete the game with a nearly perfect 100 rank score. You can only choose one weapon in this mode, sans Pistol.

-Finish the game once
-Obtain at least 9 stars
-The machine gun is found in the Apartment world in Room 102.

You just have to complete the game once on any difficulty and then save the file on a slot to your memory card to start “A Brand New Fear" file.