There are no measures that can be taken to stop what happened in California today.
Someone intent on killing with a gun will get that gun or guns legally or illegally.
They will ignore State laws and modify their guns however they want.
These killings are sadly a price we pay for the freedoms we demand to bear arms.
I personally feel responsible gun owners should be able to conceal carry everywhere anytime.
They would be the first responders gladly in these lone wolf situations.
If these lone wolf shooters thought there was a good chance of being fired back on rapidly, these massacres would stop.
California sets it up for them like shooting fish in a barrel. As do many other states.