And yet in these mass shootings are never a person who just got pissed one afternoon, pulled the open carry pistol they had at their waste and started firing.
No, instead it's the sick person who has planned, who has accumulated weapons over time, who has picked a target location and methodically executed the massacre of innocents. That contradicts your position that these very types of mass violence events would be prevented by making it more difficult to obtain firearms or ammo. A nut with a plan has patience, as evidenced by almost all of the mass shootings the US has experienced over the past decade. And with those types of people who plot to do violence, do you really think that if they cannot legally obtain a firearm they will just throw their hands up and say, "well shucky darn, I guess those anti-gun people foiled my plan!". Hell, the Aurora theater shooter even bought supplies and took the time to create pretty significant booby traps at his home which just as easily could have been used as the attack material. Was his plan detected by the very thing you say should have help find him? Nope. Creating weapons that had nothing at all to do with firearms was completed without any suspicion.