At the Mission computer press ctrl+F1 .
Then You can enter one or more of the following codes. All the codes are case sensitive !!! 
Correct codes will be confirmed by a beep. 

Code:  Result:
Redtop Trod 		 Select all single player missions from the mission 
Menu.

Half Libel 		 Activates Teleport key.
Had A Nude On 		 Activates Invincibility key.
Brass Clue 		 Activates Amunition Key.
Mother Mourn Us    Changes mountain on mission 1-5 to Mt. Rushmore with 
programers faces.

Chaste Coed 		 View the end video sequences.
Rotted Drop		 Activates Destroy Target.
Horny Elk Leer 		 Activates all targets on radar key.
O'Sarge 		 Gives Seargeant in training missions an Irish accent.
Swiss Throat 		 Changes Citadel building into 7th level headquarters.
A Scramble On 		 Let's you take screen shots by pressing SHIFT-CTRL-
RIGHT CLICK.

Range Goes Gory 	 All Levels, and lots of other stuff 
Dunk It Here 		 Recording Session Outtakes 
Oh No! Less Japan 	 Activates Ion-Strike Key

In game Control+X for Ion-Strike
In-Game-Keys :
CTRL+I			 Activates Invinciblity
CTRL+Z 		 Refills ammo
CTRL+F 		 Destroy current target
CTRL+B 		 Teleport Behind Target
CTRL+P 		 All targets on radar