Cheat mode:
At the title screen hold L, Z, C-Up, and C-Down.
Hold more ammo:
At the character screen, press C-Down, C-Up, L, C-Right, C-Down, and then press R.
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Fighting Force 64 Cheats, Codes & Walkthrough/Guide/FAQ for Nintendo 64 from Cheat Code Central
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Fighting Force 64 Cheats at CheatingDome.com.