The executive order you mention would probably stand, SC. If he did something along the lines of banning assault weapons (either their sale or possession), limiting some kind of ammunition, or unilaterally instituting some of the other kinds of gun control meastures that have been discussed, he would probably be on disastrously thin ice, i.e., he would probably lose in court (say 65/35). It would be a Youngstown analysis, and the court would additionally consider the fact that he was (a) acting in a sphere that implicated a constitutional right (second amendment), (b) in an area that fell within Congress's, not his usual powers, (c) without a very good argument for an emergency, and (d) arguably against Congress's explicit wishes. (cf. Justice Jackson's concurrence).