That’s all fine but it doesn’t really address the question. I’m trying to see what harm comes about from a (7, 14, 30 day?) waiting period. Saying those other types of guns aren’t as commonly used in mass shooting ignores the fact that all types of shootings are tragic. Self-inflicted as Brisket already pointed out. Accidental. Domestic Partner Violence. Just because these things have become commonplace and don’t make headlines doesn’t mean we shouldn’t take action to reduce them.
So in weighing cost/benefits to waiting periods, I’m seeing potential benefit not only for mass shooting but for other types of shooting. What I’m not seeing is a downside? You mention self defense, which I can somewhat see, but I don’t know how waiting periods limits someone owning a gun for self defense? If a threat is known and is so eminent that you needs to buy a gun right now, it feels like the police should be involved?
There is no single solution, but waiting periods seem like a low cost step