Im pretty sure you have to register any 80% you build due to a recent law change. You used to be able to build them for yourself with no numbers but those days are gone.
You also have to pay attention to specific laws for your state. When it comes to AR's the laws can vary greatly from state to state.
Go to some local gun shops with a list of questions and write down their answers. Go to several shops and ask the same questions because I have found even some gun shops are not clear on the laws.