I think you actually argued against yourself here. You claim it’s the availability of guns plus NRA propaganda. These two elements are the driving force, according to you. This you said started in the 70’s. But these sorts of shootings began basically in the 90’s, and early 00’s.

The question is, if these driving elements existed for longer than the shootings, is it actually causal? I don’t think so, and I think you just provided good evidence against it.