You, as an individual, can sell the firearm at some point in the future.
For my own edification, would you point me to whichever Title, Regulation or Code that covers this?
One of the great things about our nation was that everything was legal, unless there was restrictions or laws against it. Rather than requiring a law to legalize something, as is customary in some nations.
Unfortunately the last several decades have seen laws imposed on so many things that premise no longer means much.
However in this case a firearm you legaly build is legaly your property. No different than a firearm you legaly purchased from a store, or from a private party.
The problem however is that if you are building firearms with the intent to sell them for profit. That requires an FFL. So if you build the firearm planning to sell it, then you are commiting a crime. If you build it for yourself, and later sell it then it is just you legaly selling your property.
The legal concern is if you sell many homemade firearms, even if you did not intend to sell them when you built them, the ATF can still make a case against you that you were illegaly building firearms to sell for profit without a license.
So for practical reasons it is simply best not to sell homemade firearms without a license, and retain them for personal use. If rarely you choose to sell one, keep it a rare occurance.
Further, much of the code does not cover things, because a lot of what people are prosecuted for are in fact ATF interpretations and recomendations, which they enforce, but are not laws. They get a lot of discretion in enforcement of the actual statutes, sometimes creating several of thier own interpretations from a single statute, and then enforcing them as all unique statutes.
Sometimes that is within thier scope of powers, and sometimes it is not, but it is still what they require to avoid confrontation with them.
They recomend you put a serial number/ unique markings etc. That is clearly to allow firearm traces.
They recomend you sell no more than x number of homemade firearms within a given time frame.
If you do not sell homemade firearms at all you should have no problems building your own for your personal use. Follow state and federal laws.
Do not build an NFA firearm without doing the proper paperwork beforehand.
Pistols require rifling to be legal. Long arms must be 26" overall, 18" barrel for shotguns and 16" for rifles. Fires from closed bolt. Is not select fire.
State laws can be more restrictive.