we already have them. It's not hard to make a firearm, and the 3d printed weapons scene has taken off quite well. all of the good ones still need metal parts ofc, but they are pretty easy to get your hands on in many cases.
It really depends on the design and how much of it somebody is committed to 3D printing. If somebody wants to 3D print the bare minimum for legal purposes and use a parts kit for the rest, there's a number of designs that seem identical in function to something factory made.
It's not hard to make a firearm, but it's pretty hard to make a good one. I could maybe make something about ten times worse than that thing that one Aussie kid made during WWI in his garage.
specific parts. you need metal to withstand the pressures of the actual bullet to get a somewhat degree of reliability, so any pressure bearing part needs to be metal, everything else can be plastic, but the more metal the better. Now you can get some more basic designs with parts that you could fabricate at home, but a lot of the higher end designs require off the shelf gun parts.
The "leading design" right now is the FGC-9 which is actually seeing a degree of use in myanmar(??). The design requires metal parts that could be feasible to fabricate at home. However it is shockingly easy, even in heavily restricted countries, to be able to order the metal parts.