EDIT 2: Here is also a much easier to use program to allow you to make custom weapons without having to swim through a sea of code: http://forums.steampowered.com/forums... Once again, I had no part in creating this program or thread, all credit goes to bogeymanEST.
I am in no way responsible for any damage you may cause to your computer or TF2 by doing this, though any such occurance would be extremely unlikely.
Anyhow, I have created the PRD, which fires stationary rockets that act as virtual landmines. Anyone who touches them (except you) will trigger the explosion and damage them and anyone within a generous radius. They last until detonated or until you switch class and can be placed in unlimited amount, however you can only hold 12 at a time to help prevent spam without a source of ammo.
Due to the complications with what slots the demoman's weapons appear in, the PRD will overlap the grenade launcher. If you use all of the PRD's ammo, a small glitch occurs and you will get a grenade launcher with only 4 shots in it, that will disappear again once you've used up the 4 shots or gotten more ammo for the PRD.
I enjoyed making this video quite a bit, so you can probably expect more in the future!