Battlefield 4's multiplayer has 6 weapons you can unlock by playing the Battlefield 4 single-player campaign: the Shank, Machete, M412 REX, P90, M249, and QBZ-95-1.
You can play the Battlefield 4 single-player campaign on ANY difficulty to unlock these weapons. And you can replay the final mission 3 times to unlock the 3 multiplayer class specific weapons, instead of replaying the entire BF4 campaign.
M412 REX - Complete the first mission: Baku
Shank - Complete Mission 5: Kunlun Mountains
Machete - Complete Mission 6: Tashgar
P90 - Sacrifice Hannah
M249 - Sacrifice Irish
QBZ-95-1 - Sacrifice neither