At what point do you have to have a FFL to transfer items?

For instance, if I want to build a firearm. What part must be "registered" and shipped through a FFL? More clearly, could I order a barrel, stock, reciever/action, trigger all seperately and put it together myself; or does the action/reciever have to go through an FFL?


Hope that makes sense. Thanks for the help on the noobish question.