Bug Exploitation
     Go  to  Multiplayer and host a newgame. It can be any connection.
When  you  get to change the game settings, one of the menus let's you
set  unlimited ammo. Start the multiplayer game and then quit it. Load
an  old game or start a new one and you should have unlimited ammo, as
well  as  unlimited  grenades. You only have to do this once each time
you start the game.