Completion bonuses:

     Successfully  complete the game under the easy difficulty setting
to unlock Gallery 1 and the Cabal guns.

     Successfully  complete  the  game  under  the  normal  difficulty
setting  to  unlock  the  .50  Desert  Eagle  and  the hard difficulty
setting.

     Successfully  complete the game under the hard difficulty setting
to unlock the Molotov cocktail and the expert difficulty setting.

     Successfully  complete  the  game  under  the  expert  difficulty
setting to unlock the rocket launcher.

     Successfully   complete   instant  action  mode  under  the  easy
difficulty setting to unlock the Cabal SMG.

     Successfully  complete  instant  action  mode  under  the  normal
difficulty setting to unlock extra adrenaline.

     Successfully   complete   instant  action  mode  under  the  hard
difficulty setting to unlock extra health.