They can't create new law with an executive order. Let's say that Congress passed a law against all rifles with a pistol grip, but didn't define what a pistol grip actually is. An executive order could clarify that. So without specific guidelines, they could define a pistol grip by executive order to include for instance thumbhole grips, and declare that those are now illegal.
They're not supposed to, but what if they did?
What's keeping them from doing it?