If you're trying to mod guns, it's a bit harder since the latest patch. Modded guns can no longer (as far as I know) contain parts that they wouldn't usually, i.e. a pistol can't have shotgun parts, etc. Modding is now easiest done with the program WillowTree/WeaponWorkshop. In order to successfully mod, I have found that it is necessary to have the type of gun you're going to mod already. For example, if I wanted to make a modded Pestilent Defiler (revolver), I would find or buy a cheap revolver, make a duplicate of it in WeaponWorkshop, then edit its parts to make it into a Pestilent Defiler. This has, so far, worked every time.