Successfully complete the indicated task to unlock the corresponding
weapon in Multiplayer mode:

M249: Successfully complete the "Final Duty" assignment during the final 
ampaign mission.
M412 REX: Successfully complete the "Tombstone Actual" campaign mission.
Machete: Successfully complete the "A Trapped Wolf Will" campaign mission.
P90: Successfully complete the "Peace Maker" assignment during the final 
campaign mission.
QBZ-95-1: Successfully complete the "To Valhalla" assignment during
the final campaign mission.
Shank: Successfully complete the "Fang Of The Underworld" campaign
mission.