The media claims people can go online and buy complete gun kits without a serial number.
Watch how fast the government snaps into action to outlaw them.
The media claims they're "100% legal". They're probably trying to get conservatives arrested. I don't think any gun without a serial number is legal.
Actually, there are weapons without serial numbers that are perfectly legal. First, are weapons manufactured before serial numbers were standard. The second are weapons you manufacture at home. Buying a kit means buying the lower receiver, the part normally with the serial number for an AR style rifle. The blank as it is called is no more than 70% complete. You have to remove a lot of aluminum and drill the holes for the trigger, locking pins, and trigger mechanism in the blank. Normally this requires a pretty extensive shop set up, but there is a company in Texas that manufactures a computer controlled milling machine just for this.
The blank is now a completed lower receiver once you install all the parts. All you do now is install the upper receiver and barrel, the bolt and carrier, and with a little more moderate gunsmith skills you've manufactured a rifle at home. It is fully functional, and perfectly legal because you manufactured it for personal use. Note, it is legal so long as it is single shot or semi automatic only. It can take a 100 round magazine, but it takes 100 trigger pulls to empty that magazine.
Some people prefer to do this kind of work at home because they truly love to work with metal etc. Now, if you want to gift the rifle, or sell it, you have to have a serial number on it. You also have to maintain manufacturing paperwork for the rifle. Your friends can use your equipment to make their own rifle, but they have to physically do the work. With the Milling machine from Texas mentioned above, that is just touching the start button after paying for the blank.
Yes it is legal, and it is not widely done for the obvious reason. It is an expensive hobby that is somewhat limited in application. Far cheaper, and easier, to buy a finished lower receiver. That requires the ATF form for transfer of ownership of a firearm from the dealer.