It doesn't. Lanza could have used a squirrel rifle in his rampage.
He was going up against small children and un-armed elementary school teachers. He probably could have done nearly as much damage with a machete, and definitely could have done more damage with Molotov cocktails and homemade explosives.
The focus should be on arming willing teachers, custodians and staff, and on finding out why so many deranged leftist/democrats are on behavior modifying drugs and go on evil rampages... and how do we back these crazies down from their lunacy.