I don't think they have that much power with executive orders. Some quick googling suggests they can do things like influence the ATF or ban imports of assault weapons, but they can't ban assault weapons outright without Congress.
I could be wrong, but that's the info I've found so far.
Source: https://www.google.com/amp/s/www.the...n-on-guns/amp/
Even if he WAS that stupid to try, the courts would throw it out - especially right now considering the make-up of the Supreme Court.