I have seriously and honestly managed to enable god mode and unlimited ammunition in this game, however it was a bit tricky. I managed to find what code to use, but finding out where to paste it in the game files was the tricky part. I did some searching online and found out what to do.
I have the Steam version of this game, so I went into the Steam folder on my computer. I read that you have to paste the code into the user.cfg file. The problem was, I found two of those files on my PC relating to Steam. One for Metro 2033 and the other a few sub-folders down in the "userdata" folder. I put the code in the user.cfg file that was in "userdata". I read that you may have to open and close Steam for it to work and you have you paste the code in the file again if you exist and start Metro 2033 again.
Apparently when you exit the game (and Steam) then start it again, Steam checks the file you edited with a file on Steam Cloud. So your file ends up resetting to its default. That is why you have to enter the code again, or at least that's what I remember. I haven't played Metro 2033 recently.