Just add glass/carbon fiber particles and/or composite materials to the mix to get metal strength.
Or a really good engineer could come up with a resin impregnated sintered metal sort of material that becomes solid metal in a typical kitchen oven (or with perhaps a special oven - I’m no metallurgist). They use a similar process (without the 3D printer) already for some parts, I understand.
The Austin guys at Distributed Defense are working on a 3-D printed gun. However, they lack a real engineer on the team (one is an electrical engineer, they need a mechanical one) as well as someone who understands material science (so they don’t end up with a gun that melts or deforms during use).
There’s real potential with the technology, but the “print a 3-D gun” effort needs more engineers behind it before it could become a reality.
I just hope someone takes up that cause before the bill to make 3-D printing of guns illegal passes.