I just found this out today, so maybe everyone but me already knew this, yet if it even helps one person, I'm happy. Also, maybe this fits better at Model Editting or Emulator section, but I'd say this is the best forum for it.
The title might be a bit misleading: setting this up takes a while, however after that a model edit takes less than a minute.
1. Set up a private server (this is the long part, not going to explain how to do that since there are enough topics for it).
2. "overwrite" an existing gearpiece, like your weapon, with something custom and another ModelID.
3. Enter your server, and get the item with the .additem command. This will now be saved in your cache.
4. Go back to retail. Your weapon (or other gearpiece) should now have the new ModelID you entered.
This might also work for stuff like mounts, however I'm not sure.
-Aldun