Reach the indicated number of kills on the listed side in Multi-player mode to unlock the corresponding weapon.

Blue/Coalition
M4 with Red Dot Scope: 50 kills.
AA12 Automatic Shotgun: 100 kills.
P90 Sub Machine Gun: 300 kills.
M249 SAW Machine Gun: 500 kills.

Red/Insurgency
Skorpion Machine Pistol: 50 kills.
AN94 Assault Rifle: 100 kills.
Steyr AUG A3: 300 kills.
Heckler-Koch HK MG4: 500 kills.