The part of the firearm that is considered the gun, legally speaking, is the lower receiver. And a lower receiver can be used in a functioning firearm. In Europe, they don't regulate lower receivers but the parts of the weapon which bear pressure.

However, it is possible to manufacture these parts yourself - look up the FGC-9.

There are totally plastic designs for small calibers intended to shoot only a few times.

Then there's stuff like the FGC-9, with it's home made ECM barrel. Which was designed primarily for areas like europe where other firearm precursor parts are hard to get a hold of. https://www.youtube.com/watch?v=Q1Z9DpoGW7Y

At least for the AR-15 platform (the weapons pictured in the article), only the part that bears the serial number is regulated. Everything else can be bought freely and legally. For the AR-15, that part is the lower receiver.

The only part that is actually considered a firearm under federal regulations is the lower receiver, so it is the only item that is controlled and requires paperwork. Everything else (with the exception of prohibited or specifically regulated items such as suppressors) is a generally available component that does not require the ATF to approve the purchase. Laws state to state on other components may vary, such as magazine capacity.