How do you create Custom/Modded Weapons for Fallout 3?

No, I am not talking about creating a weapon with the workbenches in the game, I am talking about taking a weapon from within the game already, copying it, then editing the name and how it reacts with the world(Damage, value, weight, etc.) I know it can only be done with the PC version, but I want to give my character something like a customized revolver that does 150 points of Damage, and give it a new name, you know, stuff like that.

I know it can be done, and would be really simple if I knew how. But, how do I do it?

A link to a tutorial or a large explination would be greatly appreciated.