Lemme play devils advocate for a second. Say I just want to sell guns and damn the moral and legal reprocussions.
A 50k metal printer (I have no knowledge here so bear with me) if indeed able to print a decent (fires enough times to sell lol) Ar-15 or anything similar, wouldn't I be able to make my money back fairly quickly? *just humor me I realize that would still not be a wise way to invest in such an enterprise.
I only know about the 3d printers from what I've read with interest being in the field. So, my knowledge might be a little outdated.
Last I read, there was liquid metal and sintered metal printing. Neither will make a part to better than a +/- .003" tolerance, and that's pushing it. Gun bores are at least +/-.001", aren't they? Not sure of tolerances in the rest of the gun. Also, many parts of a gun are heat treated if I'm not mistaken. Parts coming off a 3d metal printer would still have to have a heat treat. I'm not sure sintered printing even lends itself to that.
This is a quick answer, I have to leave in a second. I'll add more later, maybe.
My first reaction would be that it would still be a too soft, loose tolerance, weapon which might fly apart upon shooting after a while. That might indeed take a little while from what I've read about advances in the field this morning, but would still be a real danger to the shooter.
10 years ago, engineers used to print plastic prototypes of metal parts for R&D. When I questioned that, they said, "Hey, these things will work for weeks before flying apart. That's long enough to test them."
So, I imagine those guns would work for "long enough for some uses", but I think a gun maven would still be disappointed.