There's nothing in the statutes about 80%
anything, that's something gun owners made up all on their own.
The ATF either rules that it IS, or it ISN'T a firearm. Part of that definition would rely on a determination of "what is required to finish this" - e.g. is it "readily made" in to a functional firearm. If all you have to do is
assembly work, or, maybe some light hand-tool work that doesn't require specialized knowledge... then yeah.. it's gonna be called a firearm.
However in this case we (nor the creator) even knows if it will WORK once it's done. There's no way in Hades the work to this point is going to be considered a "firearm" as it would take specialized knowledge, skills, and prints, along with substantial equipment and time, to complete the "soon to be, might be, might even possibly work as a" firearm.
This being said, since this is a *brand new* firearm design (even if it's scaled off something else), they're going to want to classify it. I would *expect* them to say "send us the functional prototype when it is done" which is what they *usually* do on brand new firearm designs.
Then they'll keep his gun for X months while the tech branch does their "thing" (part of which is determining if it can be readily converted to full auto; highly unlikely without serious fabrication work). Assuming all that goes well, they'll issue a letter. (I have one such letter here for a one off creation I own, that went through ATF tech labs.)
The process is long, but if you want to do it the right way... it is what it is!
On the "Markings" bit, the barrel / handguard won't do, and I retract that concept entirely. Century International got a hand-smack over that not long ago (which I wasn't aware of prior to
yesterday), and clarified that ALL markings are to be done on the receiver or frame of a gun. Period. (Evidently they realized that people were swapping barrels out on imports and, as Century was marking BARRELS, it created a bit of a legal snafu since the import marks were
lost, which created a certain tracing issue... or inability to DO any sort of tracing...).
Anyway.
Carry on.