As far as safety or security goes 3-D printing changes nothing. Those who can legally own guns can still get one, only cheaper.
That's true, for the present. Home 3-D printing is currently done in plastics. But Industrial printers are already out there, that laser-sinter powdered metals into the equivalent of milled parts, that can be heat-treated and/or annealed, and have the same mechanical properties as current commercially-manufactured parts.
That tech will show up in home/hobbyist 3-D printing relatively soon, I'd say 5 years at the most. And then a printed weapon WILL be quite possible. . . Even now, a modestly-equipped home workshop can build an AK rifle . . . I have friends who have done it. . .and fired the weapons. . .
Which is fine. Handguns with plastic receivers are pretty common.