unless we're speaking of a bomb or some such, so called 'mass violence' is no such thing. It's sequential violence...one act of violence after another after another.
The solution is to stop it fast.
If an armed citizen is present when a crazy pulls a gun (knife, hammer,) and runs amok... The citizen can put a STOP to that silliness right quick.

You don't want less guns...you want MORE. Everyone who can handle a gun should have one at all times. Kind of like smartphones..

When seconds count the police are only minutes away...