BFG (M249 SAW) in City Streets level:
Enter 5SQQ-5THA-ZFFV-7XEV as your profile name. After selecting "Done", you will be able to re-enter an actual profile name. This is how you know the code was accepted. When you begin the first level, you will have the BFG weapon.
Successfully complete the game under the normal or hard difficulty settings. All guns will now be silver and have unlimited ammunition.
Black Ops difficulty setting:
Successfully complete the game on the hard difficulty setting.
Successfully complete the game under the Black Ops difficulty setting. You will now have the M16A2 (M16 with grenade launcher attachment) on every level.