Amy, your logic is too simple....
1. If these types of weapons were not available (both banned and made illegal) it would be harder (not impossible) to access them.
2. I have no idea the nuts & bolts of using these or other types of oddball weapons are effective as his weapon. Very difficult to kill 26 people with a speeding car.
3. Lastly, not all mentally ill people are dangerous and it is impossible to predict who will carry out a mass killing.
So, the most sensible measure is gun control.
Amy, do you know why I'm not going to jump on my private jet this weekend? Because, I don't have one. Do you know why I'm not going to shoot anyone this weekend? Because I don't have a gun.