Note: We were not discussing murders in general, but mass shootings, which is a tiny subset of all murders. So I changed "murders" to "mass-shootings" in your statement which I address below.
Doesn't this tactily imply that criminals who commit mass shootings are a representative sample of all gun onwers when it comes to selection of tools for killing?
Looking into it, it would appear that "assault weapons" are not a majority of all firearms. This
article says that assault rifles are a small fraction of all firearms owned. Another
article has this:
Taken together it implies that the are more non semi-automatic weapons than semi-automatic weapons. And that most run-of-the-mill gun incidents (e.g. domestic violence, street violence) apparently do not involve semi-automatic weapons with large capacity detachable magazines.
If this is true then mass-shooters are probably
selecting semi-automatics with large detachable magazines because they they are a better tool for the job than, say, a revolver.