Ok, so start your game. Put it on the hardest mission and difficulty and beat it 3 times. After that, restart the game but this time press every button on your Xbox 360 controller 10 times in 20 seconds, right when you start your new game. Then, you sort of hack the game's script due to an unknown glitch. Now, you are going to have to press the correct buttons to get infinite ammo. See below...