I'm guessing that they're not worried about the hobbyists, who are generally more responsible; but the random people getting assembled drones as Xmas gifts.
Anything short of full-auto or under 0.50 bore (more info: https://en.wikipedia.org/wiki/Title_II_weapons). Even then, certain categories (SBRs, AOWs) can be legally owned after paperwork and filing for a tax stamp ($5 for AOWs transfers, $200 for SBRs and supressors).
This is why you see AR15 "80% Complete Lowers" being sold. You buy what is basically a chunk of metal, finish the machining (can be done with a special jig and a drill press), and you can have a functional AR15 that nobody but you has any knowledge of.
The "upper receiver" and barrel, trigger parts kit, bolt assembly, and other bits are just pieces parts and pipe, as far as UPS and anyone else is concerned. When I built my AR15 (out of a locally purchased fully complete lower), I bought the upper receiver assembly online and UPS delivered it to my front door.
Plus, not all states require registration of "normal" firearms. In Texas, the only "registration" is the ATF Form 4473 that is filled out at time of purchase, and those are held by the seller, not the government.
