I have known about the thirdperson cheat code in source for awhile. The problem is in most games for source you just end up playing while looking at the front of your person, but not portal! In portal you can do real third person, you will actually shoot where you are aiming! You can also get all of the Hl2 weapons (gravity gun!) by using the cheat impulse 101. In third person, the guns will coem out of her at a certain funny place. They will be normal in first. That cube is really bigger then you thought.
Step by step:
1. Press ~ for dev console (if nothing happens, go into the advanced keybaord settings and active dev console"
2. Type in
3. for third person type (type again to get out of 3rd)
4. for guns type (type again to get more ammo)