...when pressure-bearing components are made from thermoplastics, which has never been argued to be a well-suited use. When parts are printed from metal alloys... the game changes.
And for what it's worth, there is a Canadian inventor (who is remaining anonymous) who has successfully printed and fired a single-shot rifle.
3D printing is destroying the separation between idea or data, and fabrication, and it's doing it on an affordable, individual level. Gun control from a regulatory and political standpoint has always depended on tight control over firearm's entry points to commerce (manufacturers and importers), and compliance by intimidation of everyone else. If everybody can be a manufacturer, and it doesn't require machinist skills to make something of workable quality, centralized control is impossible.
Watch - the response is going to be to try and make possessing 3D data or plans a criminal offense. Thought crime.