I could theoretically manufacture a pistol-caliber carbine myself for under $1000, including the 3d-printer. As in, from bar stock + hydraulic pipe + DC current source + 3d printed components. Look up the FGC-9 - it's pretty impressive.

That's with current tech, and I don't think the tech of 20 years from now is going to be worse. If anything, 3d printing will be even better.

Now, that FGC-9 isn't nearly as concealable as a standard Glock. But I'm sure there's ongoing efforts to make a home-manufacturable semiauto pistol, too.

The more interesting thing to manufacture yourself is ammunition - smokeless powders are rather flammable, primers take that and scale it to 11 with a side of toxicity, and case manufacture has a lot lower tolerances than you'd think. (Bullets are comparatively easy, if you have a source of lead stock and can make a mold) But the number of guns in circulation in the US pales in comparison to the amount of ammunition stockpiled - estimates generally put annual sales at 8 to 10 BILLION rounds of ammunition. A lot of that is fired each year, but that's still a hell of a stockpile to go through first.