I know that, it's a federal deal, specifically a last minute amendment to the Firearm Owners Protection Act, of the mid 80's. IIRC 1986. And it's in blatant violation of the second amendment, The previous "tax based" restrictions (National Firearms Act) were too of course, but at least it wasn't a complete ban. Supreme Court has never ruled on either law. Never.
However you said:
Try buying a rifle like the troops carry (or even a defanged one that shoots one shot per trigger pull.)
I was responding to the part in bold. You can buy a semi-auto only clone of the M-4, a similar version in 7.62x51, or a SA clone of the M16A4. Complete with bayonet lug and flash hider. Just not the burst fire capability.
That's not that much of a loss, sustained rate of fire with burst is about 90 rpm, in semi-auto, it's 45 rpm. (M-16/M-4).
Except for the part of the NFA that applies to short barreled shotguns, and then the ruling was based on no evidence having be presented as to the military utility of such weapons. I think even the Supreme Court would have allowed taking judicial notice that a Thompson or a BAR had such utility.
It's really too bad Miller and his buddy didn't have one of those, as more notorious criminals of the time did.