If we were probability driven, we wouldn't care about these mass shootings. We'd be focusing on other things, among them Chicago.
As cheeks points out, the slower rounds thing is a non-issue.
The best point you make relates to magazine size. But, in the end, I'm not sure that will do much. And, so few people are killed by these events, it makes sense to solve other situations first, and just let people defend themselves.
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