Iirc, and don't rely on my memory for anything real without consulting a local attorney, as long as the firearm isn't functional, as in it has been rendered unable to fire at all, regardless of being assembled or not, it's no longer a firearm fur that specific purpose. But there's fairly strict rules about what actually renders weapon legally fucked enough to pass muster.

That being said, it isn't really about US laws for the most part, it's state laws that will fuck you in the ass. So, again, grain of salt.

But that's how people can own actual military gear without an ffl. You get a "machine" gun that's entirely welded into a brick, and has paperwork with it, you don't have to do anything else.

With another that being said, a convicted felon is going to want to be very careful anyway. Yeah, eventually it'll be handled and it won't come back on them, but it's still gonna be at least a brief clusterfuck if the wrong person sees it. A lot of cops don't give a fuck and will cause trouble before checking to see if it's actually legal.

Edit: it's usually the receiver that's considered the important part. And it isn't enough to just not have one visible in your scenario. You'd want proof that there wasn't one, just to avoid hassles