The AR15 is unusual in that the serial numbered part does not include the barrel, the chamber, or the bolt. The serial numbered "lower receiver" holds the trigger mechanism, holds the magazine, and allows attachment of the stock and the "upper receiver".
That means that the 3D printer really just needs to be able to make the simple part. The parts essential to firing the cartridge are available without restriction. (At least for now.)
This implies that if you make the lower receiver (the part that gets the serial number, which is the "firearm"), and then buy the trigger group and upper receiver and install them, then in theory it should be perfectly legal.
Perhaps the upper receiver will become the serial-numbered part, as is the case in many other "2-piece" rifles such as the FN-FAL.