The hard part of all of this is primers. Virtually everything else is straightforward (propellant is hard, but not as hard as primers. Reloading from infinite primers and powder is essentially something a one-armed blind person can do with a reloading press.
Most of the parts of guns are not under much mechanical stress, and the parts which are, tend to not be the regulated items. A convicted felon could buy commercially made barrels all day long (they're not firearms or in any way controlled), which are probably the firearm part with the greatest difficulty of manufacture, along with springs.
Of course, one can manufacture a gun from a 3D printer plus some additional stuff. But there is a qualitative difference between 'click print' and you get everything you need and a project to get everything you need to build a gun, which utilizes a 3D printer ( and some chemistry and some additional metalwork). After all, it is today possible to manufacture a (completely unregistered) gun yourself utilizing a CNC cutter and some craftsmanship.
Most of the parts of guns are not under much mechanical stress, and the parts which are, tend to not be the regulated items. A convicted felon could buy commercially made barrels all day long (they're not firearms or in any way controlled), which are probably the firearm part with the greatest difficulty of manufacture, along with springs.