As another person commented somewhere in this thread, the availability of weapons is at a low point historically. Back in the 1930s, a person could order a machine gun in the mail and have it shipped to their house. Until 1986, people could purchase new machine guns at their local shop after a good amount of paperwork.

You're correct that banning semi-autos would lead to reduced deaths in mass shootings, but it's just putting a bandaid over a greivous societal wound. I don't feel that enough thought is put into why people are going on suicidal rampages against children or minorities, there's just a "people be cray" attitude then they push for disarmament.

Without addressing that societal problem, I just see weapon control becoming more and more stringent in response to the unsolved problems in society. Banning Semi-autos today may reduce deaths, but it'll be lever-actions tomorrow, then bolt-actions, then knives, then vehicles.

If I were dictator, I would temporarily add semi-autos to the NFA list (along with giving them the resources to process applications promptly) to stop new sales and transfers without stricter checks. Then I would put effort into determining the causes of those rampages and fixing them.

Male socialization, political radicalization, and media contagion are three factors I think lead to these rampages, and to merely remove guns from the situation is the societal equivalent of locking someone in a padded room and declaring the problem solved.