You can build all the firearms you want for personal use. You'll get in trouble if you start building and selling without a license.
As long as you possess them and keep them in state, they don't need serials. If you go to sell them, they must be serialized.
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If you are handy you may want to look into the 80% lowers but you will need drills , Taps and a drilling jig. All that will run the price of your lower to almost the price of a complete rifle. And i do not know what the legalities of buying an unnumbered lower then finishing it into a completed firearm would be as somwhere along the way it has to be serialized I think.So the BATF&E would get there hands in your pockets anyhow.