Just because it's not possible now with the early generation of 3d printers available to the public, doesn't mean the technology won't catch up to the idea in the near future.
I don't think the printing technology is what you have to be worried about
It's all about materials
If a non-metallic material comes along that can be used to make a functioning gun then you have something to "worry" about. It's unlikely that that specific material would be able to be "printed" and maintain the proper characteristics, but if it could then there is something else to worry about, but that's a gigantic step forward.
Take steel for instance....all steel isn't created equally. The crystalline structure of steel is changed during the melting, cooling and even during forging that gives steel the proper characteristics for whatever function you want to use it for.
Take a knife for instance. You don't just pour molten steel into a dye and voila, you have a knife(....well, I guess you could, but it'd be a really shitty, brittle knife that wouldn't hold an edge), you have to heat treat it, quench it, forge it, heat treat it again...and probably several other steps before it becomes a knife. The point being, if somehow a 3-D printer could print steel, it's not simply a matter of putting little bits of metal in the shape of a knife. For a gun it wouldn't be much different (yet it would be more complicated), you don't just have some kind of metal being put into place, it has to be a specialized metal, formed in the right environment and in the right way....something that I seriously doubt 3-D printers will ever be able to do....at least on a scale that would be available to every Tom, Dick and Harry.
Same goes with any "futuristic" non-metallic gun worthy material. It's very likely not going to be something that you can put together the way a 3-D printer would whilst still maintaining the necessary properties, as it's going to be something formed under very specific conditions that likely require very expensive, sophisticated systems, machinery and know how.
All that being said, there's nothing wrong with debating the future, but you don't want to get to far ahead of yourselves