It probably behooves all of us to understand what an Executive Order is and what it can and cannot do:
https://www.heritage.org/political-p...ecutive-orders
The long and short is - absent a federal law allowing them to regulate semi-automatic weapons and large capacity magazines, a president would not have authority to issue an executive order banning them. Worst case scenario is @DanM suggests, since POTUS is empowered to oversee foreign trade, they could ban the importation of firearms via EO.
But currently, no law exists allowing the president to regulate magazines or semi-automatic weapons (NFA weapons, use, but not semi-automatic). Attempts to pass a ban via EO in this way is likely to result in the Supreme Court overturning it, because there is no law allowing the president to regulate such things. And it is certainly not enumerated in that pesky thing called the 'constitution'. Which I know a lot of politicians haven't read, but fortunately, it seems like the bulk of SCOTUS and COA Justices have.