The ATF just Tweeted a link to a Q & A .PDF about 3-D printed guns.
I put a copy here.
The short version is:
Yes, it’s legal without a license as long as it is not an NFA item. If it does not have a rifled barrel then it might be classified as ‘Any Other Weapon” which is an NFA item.
Summary of who can and how to get a license to manufacture.
It seems a reasonable response once you suspend disbelief enough to allow that the ATF is more constitutionally permissible than a BRAS (Bureau of Religion, Assembly, and Speech).
Update: There are some questions (and this is why I was deliberately vague above) about what constitutes an AOW. From 26 USC § 5845:
The term “any other weapon” means any weapon or device capable of being concealed on the person from which a shot can be discharged through the energy of an explosive, a pistol or revolver having a barrel with a smooth bore designed or redesigned to fire a fixed shotgun shell, weapons with combination shotgun and rifle barrels 12 inches or more, less than 18 inches in length, from which only a single discharge can be made from either barrel without manual reloading, and shall include any such weapon which may be readily restored to fire. Such term shall not include a pistol or a revolver having a rifled bore, or rifled bores, or weapons designed, made, or intended to be fired from the shoulder and not capable of firing fixed ammunition.
It is my NON-LAWYER reading of this that since neither the current nor WWII Liberator fires a shogun shell they do not qualify as an AOW even though the barrels are not rifled.