Untraceable in a legal/administrative sense, it could still be traced using forensics. In the US the lower receiver is the component that is legally considered

Untraceable in a legal/administrative sense, it could still be traced using forensics. In the US the lower receiver is the component that is legally considered a firearm. They're required to have serial numbers and are subject to the same laws as a fully functional gun. The other components are just hunks of metal in a legal sense and may be sold and manufactured without restriction. If you are able to fabricate a lower receiver by yourself you can buy the rest of the components you need to assemble a fully functional rifle without any records of its existence or creation. According to the BATF a block of metal doesn't become a lower receiver until it is more than 80% complete. This led to the creation of a market "80% lowers" that can be legally sold without restriction and completed by the buyer. The hardest milling is done by the manufacturer and they are designed to be completed by inexperienced users with common tools. There's even a company that will sell you a CNC mill preprogrammed to finish milling the receiver.