>It's not clear to me if a rifle part printed from plastic at home is usable.
It wouldn't surprise me if it were perfectly functional. The lower receiver on an AR houses the trigger group, buffer tube, magazine, and grip. Not really a lot of high-stress parts there. The barrel, bolt, gas tube, etc that have to withstand 60k+ PSI of pressure are all part of the upper receiver.
Legally, only the lower is classified as a firearm in the United States (meaning they have to be transferred through a licensed firearms dealer if shipped); the upper and the rest of the gun are simply parts and can be shipped directly by UPS/FedEX/USPS.
It wouldn't surprise me if it were perfectly functional. The lower receiver on an AR houses the trigger group, buffer tube, magazine, and grip. Not really a lot of high-stress parts there. The barrel, bolt, gas tube, etc that have to withstand 60k+ PSI of pressure are all part of the upper receiver.
Legally, only the lower is classified as a firearm in the United States (meaning they have to be transferred through a licensed firearms dealer if shipped); the upper and the rest of the gun are simply parts and can be shipped directly by UPS/FedEX/USPS.