Successfully complete the indicated task to unlock the corresponding
special weapon:

Brayko's SMGs: Kill Brayko.
Brayko's stereo: Spare Brayko.
Gold assault rifle: Sneak up on Al Samad Lieutenant without any
alarms being sounded.
Gold SMG: Assassinate Brajka.
Marburg's pistol: Bait Marburg into fighting to the death in the
museum.
Omen's shotgun: Kill Omen Deng.