The only cheats there are are:
City Streets level-Unlock BFG (M249 Machine Gun) Cheat
Enter any of the following codes as your profile name. Make sure you include the - symbol when entering the code (it's next to the ! symbol). After entering the code and clicking "Done", you'll be able to enter your real profile name. This is how you know the code was accepted. When you begin the first level, you'll have an M249 machine gun instead of your normal gun.
Password/ Effect
5SQQ-STHA-ZFFV-7XEV Unlocks BFG gun for City Streets level
HQ6G-ZP3B-C5LE-WMXA Unlocks BFG gun for City Streets level
EG4P-ZGUJ-6SQJ-3X68 Unlocks BFG gun for City Streets level
BFG Code
Enter the following code to enable access to the BFG.
Password/ Effect
FG6S-WFZG-7MDP-PZGT Unlocks BFG
Unlock the BFG for Veblensk City.
Enter the password as your profile name, including the hyphens.
Password/ Effect
4SUD-A2DX-8EBU-KFFX Unlocks BFG for Veblensk City