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.