So you're telling me that you honestly believe all of these rampage killers just up and acquired their firearms on a whim, and immediately went and murdered people?
I'll indulge this when you show me where all these people are buying firearms and immediately killing people. This is a scene from Terminator 1, not a real thing that happens.
Fact- most criminals that use guns, owned them for some time or acquired them by unknown means LONG before they used them for malicious acts. Most high-profile mass shooters spent weeks, if not months planning their rampages.
Waiting periods only existed because they had to mail off the form to the court house. Now, it's instant. Literally takes thirty seconds to get a background screening, even at the court house. You don't even need all the information you fill in on the background check, just the Social Security number.
Waiting periods do nothing- oh, wait, yeah they do.
See, a gal I dated had a psychopathic ex (well, more like a dude she dated a few weeks that was actually just really unhinged and didn't take rejection well). She had a restraining order, which is just some words on a sheet of paper. And she came home to find her dog stabbed to death on her sofa. There were threats. She eventually decided she needed a gun, because the fucking cops took half an hour to get to her place. Had she not been able to acquire one the day after her dog was killed, she'd have been unarmed in the home when this psycho tried to kick the front door in.