Firearms are not well suited to 3D printer technology.
Rather then this point being a matter of debate it is quite straight forward to 3D print a barrel (for safety sake say a 9mm Kurz) and fit the barrel to a legal gun. Fire one round into soft backstop and then examine the inside of the barrel, the case and the recovered FMJ bullet. Bullet will be nearly unmarked, barrel scored and the chamber distorted, and the cartridge case ballooned at the mouth.
One round using the best 3D compound the manufacturer has to offer. Just ONE!
The point being other processes will have to be used to make viable weapons. Such equipment has been available to all Americans and even the occasional Scot for the last three hundred years.
The miracle 3D printer that can make replacements for high performance objects like firearms, cutting tools, eyeglasses and watches and do so for costs LESS then these things made by conventional processes is very much in the future.