You are correct, but there are probaly less than 10 multi-50s/miniguns or anyting simliar in the whole US (in the hands of civilians, so they will NOT be available/effective for anything. It will boil down to a man protecting his country with a small arm, with occasional overwatch from a larger caliber small arm, maybe some commandeered equipment/loyal troops from Guard or Reserve units.
God Bless
MOLON LABE
You'd be surprised.