I was discussing what solders carry as a primary weapon but since you brought it up why don't "mass shooters" regularly use chemical weapons now instead? Could it be because they're regulated and not readily available to civilians?
Then when you trial date comes up it all goes on again - so these people see the fame that others have gotten - and how people have loathed them and feared them, and their shots are heard around the world.
Not so much the case when someone mixes bleach and windex.
Fortunately that means they use guns instead of more dangerous weapons.