The second amendment states: "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

Gun nuts have managed to corrupt this wording, from a time when all free men were expected to serve as militia (army) in time of war and arms included swords and single shot muskets, into the right to possess weaponry that should only be available to persons serving in the military. 

In this instance the gun used was one which should reasonably be available to civilians. As opposed to the military grade weaponry used by the Las Vegas shooter. Which why there were only three dead in Thornton.