Make the sale of firearms that can hold and fire more than one round (bullet) illegal. The new law should be modeled after the law that prohibits the sale of fully automatic weapons to the public.
It is the ability of the shooter to fire 6, 8, 10, 100 or more rounds with out the time it takes to reload that causes the mass casualties we are seeing today. It is the ability to fire hundreds of rounds in a minute instead of just a few that makes this type of change an imperative.
This new law would not infringe on anyone's right to own or keep or legally use a gun. You can own all of the guns and ammunition you want, as you can today.