The 1989 ban wasn't enacted through executive order.
You are, of course, correct Tom.
My point, which was obviously not made clearly, was that the 1989 ban was a result of an Executive branch 'reevaluation' of existing law (the 1968 GCA). The ban was imposed based on this reevaluation of whether military style semi-autos had a sporting purpose.
Bush never signed an EO (that I know of) to accomplish this, but my point is that such an action would be well within the power of an EO, since it happened entirely within the Executive branch.
Sorry I wasn't more clear.