Enter the "Multiplayer" menu. Select "Unlockables", then enter one of the following codes.

FN2000
Enter h4h4th3r1snoc0d3 as a code.

M60 machine gun
Enter try4ndrunf0rcov3r as a code.

QBU88 sniper rifle
Enter your3mynextt4rget as a code.

USAS12 shotgun in multi-player mode
Enter aw3somef1rep0wer as a code.

Uzi
Enter cov3r1ngthecorn3r as a code.