When the full info is released, it will probably show the gun was stolen or black market. In all the history of the world, attempts by governments to confiscate all privately-held weapons have failed. I give you the American Revolution, the Warsaw Rebellion, Israel's war of independence, and the current unpleasantness in Afghanistan. Not to mention the high shooting homicide rate in Oakland, CA.
And if he had not been able to find a gun, he would probably have built a bomb.
While I see Palin's target map as immature and just what I would expect from someone who hunts wildlife from a helicopter, I really doubt that the shooter knew about it.
I am heartbroken about the victims caught in the middle, especially the grade school girl who seemed destined to be Someone Special. The shooter probably changed history for the worse when he killed her.