To get infinite ammunition, reload before you run out of shots on one clip.
To unlock bonus level, complete the game with the best ending.
Use the following cheats at the indicated screen. First, hold Left Shift and press Minus on the KeyPad, then release the keys and enter one of the following codes:
Save game (gameplay): savegame
End current mission (gameplay): endmission
Unlock all missions (main menu): campaign
Show elevations on map (main menu): topography