For someone trying to bypass firearms laws, the important part is whichever one is legally deemed the "firearm", usually the receiver. You can buy barrels, recoil springs, magazines, grips, sights, and all sorts of other fiddly bits as spare parts, which are legally no different than a spare tire for your car. If you designed a 3d-printed receiver that worked with existing spare parts, you've worked around those pesky laws. (I personally find that law, at least, to be quite reasonable, but some people seem to want to work around it as a matter of principle).
How is followign the law trying to bypass a firearm law? While making for your own self is legal, it is NOT legal if you are a "prohibited person" when it comes to firearm ownership.
And of course, to the person who's actually interested in shooting guns, rather than writing angry comments about them on the internet, the important part is whatever breaks most readily on your particular gun and needs replacement. I expect historical firearms shooters would be quite interested in being able to print parts once considered disposable, or which frequently are damaged, like clips. Or better yet, print brass casings for all those guns whose cartridges are no longer produced. There are many, many guns in collections that can't be fired not because they are old or damaged, but because the ammunition is so scarce. (There are many more problems than just forming the brass, obviously, and I don't think 3D-printing is a particularly good solution for it, but maybe I'm wrong and 3D printing will eventually help).
Forming brass is trivial if there is a suitable parent case. Suitable can mean "same diameter rim and case head size and perfectly straight". Of course you need accurate case dimensions but you can cast the chamber and take measurements if you have something totally unknown, and then have a custom set of forming and reloading dies made. Unfortunately, the really weird stuff is primer dependent - the various obsolete rimfire rounds (44 rimfire for old Henry rifles, etc) or pinfire rounds.