Super harsh prison terms with no chance of parole for "responsible" gun owners whose guns are left for their kids or crazy relatives to use.
Like the guy who does keep his guns in an useable state in a safe, and has his kid who he didn't know was capable of anything like this (like most parents of these people). Only to get a call while he's at work one day, to find out his kid had snuck into his room on night while dad was sleeping, borrowed the keys to safe, and then shoot up the school.
You probably didn't hear about it, but there was a kid who committed suicide at his middle school this year. He got into his dads safe, when his dad had mistakenly grabbed his moms keys on his way to work that day. He bleed out in sh!tter stall, around noon, it took the school till 11pm that night to find him. But hey, lets throw the dad in prison for it.