1 weapon mode:
To get 1 weapon mode, you have to beat the game once in Brand New Fear File with any ending in hard mode and get 10 Stars.
All Weapon Mode:
Beat the game with any ending on hard with the 1 weapon mode.
Eileen's Sub-Machine Gun:
To unlock Eileen's Sub-machine gun beat the game in hard in a Brand New Fear File and get at least 9 stars. If done correctly: You can get the Sub-machine gun in your first visit of the Apartment World in room 102.
Beat the game once in any mode and Chainsaw will be available at the Forest (First) in the area where there's a car.