Call of Duty: Modern Warfare: Mobilized

Successfully complete Campaign mode.

Score a kill with all gun models. The shooting gallery allows you to test out all of the available guns.

Select "War Room" at the main menu, then select "Options". Enter Select, L, R, Select, Y, Y, X, R, L, X, R, Y as a password to unlock Survival mode for your current game session.

Successfully complete the game under the Hardened difficulty setting.

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.

Successfully complete Campaign mode under the Regular difficulty setting.

Successfully complete Campaign mode under the Regular or Hardened difficulty.