And that is the funny thing about U.S. law, specifically the NFA, it was written and enacted a good eight years before the concept of an assualt rifle was dreamed up, atleast the current definition of firing an intermediate power cartridge.
Yep!
The NFA cares nothing for the power level of the round fired (until you get to "destructive devices"). If it goes full auto (including only burst fire) then it is a machine gun. Doesn't matter if we class it as a submachine gun, assault rifle, belt fed, light, medium, or heavy, all the NFA sees is a machine gun. They don't care if it is belt or box magazine fed, either.
The fact that an assault rifle has a select fire capability makes it a machine gun, and all machine gun laws apply. We use that feature, along with the intermediate power round to classify it as an assault rifle, as opposed to a light machine gun or submachinegun.
Let's say you have a pickup truck. There are certain features that make it a pickup truck. And its legally registered as a pickup truck.
Chrome wheels, extended mirrors, brush guards and roll bars dont' make it a pickup truck. 4WD doesn't make it a pickup truck. But they are features a lot of pickup trucks have.
Now, tell me you would be happy with an extra tax/special registration because your truck had foglights AND custom wheels. (combination of "evil" features) and if you didn't comply you would be breaking the law.