They tend to lean on whether it is reasonable that the Founders might have had access to such a weapon with their technology. Machine gun is just a rifle with automatic rechamber. Not an unreasonable upgrade for 1700s technology. Maybe, I dunno; political people don't have to actually care about the details.
There are limits. And if cases like this made it there they might rule that no Founder was smelting the materials. That they would have had to collaborate, in some "market dictates options" ruling to limit hermits going in a rampage. Also everyone a weapons assembly line in their home is anti-corporate capitalism.
"George Washington understood the value of civic life and sound economics! He would not have tolerated such insular selfishness! He did not make his own weapons! He engaged in trade!"
Not saying it's realistic but politics is not never controlled by people living in reality. Making shit up seems as reasonable as anything.
This is largely machine guns and explosives. Pistols, rifles, etc are ordinary weapons in common use*
*NYC authorities may not agree