Go to the unlockables in the multiplayers main menu and than bring up the enter code window and input the following codes to unlock some weapons

Code	Effect
aw3somef1rep0wer	USAS12 shotgun in multi-player mode
h4h4th3r1snoc0d3	Unlocks FN2000
try4ndrunf0rcov3r	Unlocks M60 Machine Gun
your3mynextt4rget	Unlocks QBU88 Sniper Rifle
cov3r1ngthecorn3r	Unlocks UZI