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.

You can get parts or parts kits (random example) and make the receiver/frame (whatever the legal "gun" part happens to be on a design) yourself.