It would be very different if the ability to manufacture a thing verifiabliy happens before said thing is banned.
1. "Oh these 1000 mag tubes and 50 AR15 lowers? My cat sat on the "Print" button - years ago, before the ban. Oops silly cat"
2. Despair working against those who would otherwise support. Would the casual participant so ardently want a ban on something a "criminal" could really so easily reproduce?
Going off on another tangent. . . with viable accessible 3D printing we would face a renaissance of incredibly cool designs and the ability to trivially digitally share them. The guns you buy are designs limited by turn of the (19th) century machining, or later stamping, casting or mass production molding. The next set of designs based on "printable" enthusiast boutiquey one-off components is in front of us.