And as long as the firearm does not "enter the stream of commerce" it does not have to have a serial number.
If this ever reaches the point of being economically feasible, I can see the internet flame wars now-
YOUR build is trash because YOU ran the software on a PC/ Apple/ Ipod. YOU are using LINUX instead of DOS. Sorry, you used a Chinese printer- your gun is not 922 compliant. MY software came from DPMS, and it is just as good as your ArmaLite software, and a lot cheaper. Oh yeah, well, HE used open source shareware, it is not milspec, and does not have a dust cover.
Since this deposits mini dots of plastic for the build, there will be the "old school" AR fans that swear it has to be aluminum- and think AR means Aluminum Rifle.
On the other hand, when someone makes a 1911 that is plastic based, the Glock and 1911 fans will unite (except for the owners of steel guns that will decry the heresy)
And Frank Lautenberg and Barbara Boxer will enter an amendment to an agriculture bill that will make it a crime to fail to secure your printer, and require a background check for printers sold at computer shows. And protest the cheap internet sale of printer templates- especially the ones that have the thing that goes up over the barrel.