"What do the perpetrators of the massacres at Sandy Hook, at Aurora, at Orlando, and at Sutherland Springs have in common? They were all men under 30 and they all used versions of the same kind of firearm, the AR-15, the semi-automatic version of the military's M-16 and the bestselling gun in America. It might be difficult to make this connection because as I write this, the section on the use of AR-15s in mass killings has been deleted from Wikipedia..."