BFG in first level:
Enter 5SQQ-5THA-ZFFV-7XEV as your profile name. Include the dashes. If entered correctly you should be able to enter a new profile name. You'll start the first level with the BFG.
Beat the game on normal or hard difficulty. All guns for that difficulty and below will be silver and have unlimted ammunition.
Black Ops difficulty:
Beat the game on hard difficulty.
Beat the game on Black Ops difficulty to unlock the M16A2 on any difficulty setting. It's an M16 with a grenade launcher attachment.
Thanks to Revolution Reader SaleTheMan, Dcac!