It's VERY clear that making a firearm for personal use, as long as it is not a type of firearm regulated under the National Firearms Act of 1934, is perfectly, 100% legal under Federal law, there may be state restrictions. There is an entire small industry around what are called "80% receivers", a receiver blank that has had less than 80% of the machining operations performed to produce the final, working receiver is not considered a firearm under Federal law and can be sold without restriction, people buy the 80% receiver blanks and perform the final operations to produce the working firearm (for US law purposes, the receiver is the part that normally has a serial number and, if commercially manufactured, must be manufactured by a licensed manufacturer and distributed through a licensed dealer, all other parts can be bought and sold without any paperwork) at home using a mill, a drill press, or if particularly masochistic, hand tools (for an AR-15 receiver, some types of receivers, including for an AK-47 pattern rifle, are folded and welded sheet metal). A firearm built for personal uses doesn't even require a serial number, although the BATFE highly recommends it, if nothing else to avoid problems with local law enforcement who might confuse a firearm that has never had a serial number, which is legal for one built for personal use, with one that has had the serial number altered, which is a crime.
For NFA firearms, other than machineguns (new manufacture of machineguns for civilian use has been prohibited since 1986), a person just needs to go through the process of obtaining a Federal tax stamp (background check, local law enforcement sign off if not incorporated, payment), which currently takes about six months, before starting manufacture. NFA firearms include rifles with a barrel shorter than 16", shotguns with a barrel shorter than 18", silencers/suppressors, firearms that do not look like firearms, machineguns, and firearms such as a shotgun-pistol (short-barreled shotgun with rear pistol grip instead of a stock), these firearms require a serial number and identifying manufacturer information, if an existing commercially manufactured firearm is converted to an NFA firearm, a second set of manufacturing information and serial numbers gets added.