Executive orders are bad enough. But to issue them regarding a constitutional right, where there is a distinct divide in public opinion, is tantamount to an act of war. He'd essentially be saying that regardless of the fact that the Supreme Court has ruled time and time again that the right to keep and bear arms is an individual right, that he has the unilateral power to determine how that right manifests for the people, without even involving the legislative body.
That is an act of war.