It has to be an adjustment to a current law that is in effect, an Executive order cannot create a new law, all it does is clarify and makes adjustments to how said laws are to be executed. At least that is how I understand it.
As by the OP Im sure you know Im not that good with the law .
I'm sure there is a law out there that can be adjusted to do just what they want . I don't know much about it but what about the original federal gun legislation making it illegal to own firearms that are full auto and barrels under 16" . Can they just piggy back someting on to that law ?
This is all going to come down to wording and how the wording is interpreted . My guess is they will try to get what ever law past they can and let the NRA fight it in court .