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Dreamcast - Unreal Tournament cheats

Pause the gameplay and enter the following cheat codes:

Refill health
Press Down, Down, Down, Left, Up, Right.

Full ammo:
Press Left, Right, Left, Right, Right, Left.

Invincibility
Press Left, Left, Right, Right, Down, Up. 

Level skip
Press Up, Down, Left, Right, Right, Left.

Unlock Single Player Challenge Mode 
Finish Deathmatch, Domination and Capture the Flag modes 
on any difficulty level.

Untested console cheats:
Note: You need a Dreamcast Mouse, and Keyboard. 

Press "T", after hold the ESC key, then press any letter 
on the Keyboard. Now type the following cheat codes:

allammo - 999 ammo for all guns 
behindview 0 - returns you to regular view 
behindview 1 - lets you see behind yourself 
fly - lets you fly
ghost - Go throught walls 
hidehud - Disable HUD
loaded - Random weapons 
suicide - Commit suicide