Of course there is a legal way, and it is the same way that the UK and Australia has to ban private ownership of gun: change whatever is blocking the ban:
The Constitution | The White House
In spite of the mass shootings in El Paso and Dayton, the future of gun control looks grim. Yet the lesson of the gun-rights fight is that no victory, or defeat, is permanent.
www.newyorker.com
If that means constitutional changes then someone needs to put their neck in the line and start thinking g of how, although I know it’s already been done by some groups. If it’s not outright banning then at least higher capacity and faster fire rate weapons need to be banned.
UK and Australia are civilised enough countries to know 1 mass kill with guns is too much. So don’t even try to use a count as a barometer.
Gun ownership is dumb.
Proponents of 2nd amendment are even more dumb.