They're not violating the Constitution with regards to machine guns, so you can stop right there.
"where they are attempted to regulate "parts" which the law, and their own regulations, define as "non guns".
Stewart wasn't about his sales of the .50 cal. kits. So what are you referring to?
"If they can do that, they "ban 'em all" and declare "turn them all in Mr. and Mrs. America"
They don't have the votes.
Once things get to the Courts, votes don't matter.
If Congress can pass a law as they have, that infringes upon one class of arms, then they can pass a law that infringes upon another class.
Then some bureaucrat comes up with a ridiculous test that includes things that are not guns as guns, or they come up with a test that changes the category of a gun.
For example, the M-1 garand, or M-1A springfield, or M-1 carbine all have hammers with hooks on the back. If the hooks are ground away, they will slam fire. If you can grind away the hooks with a dremel moto tool, did they make it into a fully automatic weapon? Does that make all M-1 garands fully automatic, by the ATF standards?
The M-1903 had a "petersen device: made for it that converted it into a semiautomatic that shot pistol rounds. Are all bolt actions fully automatic because a similar device could be made for them?
The tests, which are the substantive part of the law, are not written by Congress, but by a self serving "tax agency" that is more interested in controlling behavior than collected taxes. It is kind of amusing that the IRS makes money for the federal government. The Firearm part of the ATF loses money every year. Hell of a thing for a tax agency.