Pretty sure the constitution says every man has the right to bear arms, not guns specifically.
Ban guns and carry round swords instead.
If a bloke goes psycho with a sword he's far more easily disarmed than a bloke with an assault rifle.
So no more mass shootings and everyone gets to keep their right to bear arms.
-
Why not just use dueling pistols? If you want to kill a whole bunch of people then you just need to duel them first.