So. I keep harping on printable guns, right?
BAM! Functional printed AR-15 magazine. [UPDATE: Here's a direct link to the Thingiverse post.] Including an ABS spring. Its designer doesn't expect it to hold up to much use, but that's hardly the point when you're talking about the feasibility of using civilian disarmament to disarm criminals.
The article links to another designer who's uploaded a file for printing an AR's lower receiver. For those keeping score at home, the lower is the serial numbered part of an AR; under US federal law, that part and that part alone is the "firearm", subject to all the restraints on commerce and background checks that US gun control is made of. Everything else--barrels, triggers, sears, extractors--is a part or accessory, and can be sold over the counter or even mail ordered. Print this part, and you can build a functional AR around it with no records or background checks.