Good point about guns being available nationally can move anywhere, but we do have 300 million guns in the U.S. and the Second Amendment. We have to take a more "where do we go from here" approach.
At least in these mass murders, those people are mentally unstable and one way or another they will find a way to kill others. Not having a gun handy isn't going to stop them. Most of the time they put great effort into their attacks.
SCOTUS has been quite clear about Americans having a right to own a gun, but I personally believe in a sensible approach to firearm restrictions. Some are restrictions are needed, no doubt.