I believe that makes it a rifle in ATF terms. Rifles have to have a 16" barrel, or they're short-barreled rifles that require a whole bunch of extra paperwork and some taxes. The class is called NFA Items, and includes fully automatic weapons. I.e. making that thing is the same as home-manufacturing a machine gun to the law.
I'm going to hope for his sake that he's got the appropriate FFL licenses to allow him to make such things. It seems like they're really into the space, so it wouldn't surprise me. I don't think those licenses are crazy hard to get, but I could be wrong.
You don't need an FFL to make NFA items. You just need to have the tax stamp before you start if you don't have an FFL SOT.
That's good to know, I appreciate the correction!
The licenses are a massive pain to get. End the NFA.
"Braces," like the sig pcb are workarounds to these arbitrary rules and evidence of that fact.