any frame you buy has to be threw FFL!
That's why he said 80% receiver. An 80% reciever isn't finished, so it isn't technically a firearm frame or part, yet. You buy the 80% receiver, which does not require an FFL transfer, and finish the machine work yourself, then assemble it with the other required parts. Assuming you build it to clone an otherwise legal weapon, you end up with a perfectly legal pistol/rifle (by federal law, your local laws may vary) that is not registered in any government database. Obviously building an 80% receiver that is fully automatic is still just as illegal as converting an existing firearm.
is one source for purchasing these.