The military rifles today usually don't have "full auto" beyond three shots, because anything after that becomes less accurate (aside from real machine guns). We had one tragic shooting with bump stocks, and I'm fine if they are prohibited, but it's not as if this was turning into a common situation. Maybe it would have?
A lot of mass shootings we read about take place at close range as opposed to some guy with a rifle situated above everyone, they are in a school or nightclub etc. where a pistol and/or shotgun would be deadly as well and probably preferable to any rifle with any length.
I am bit curious why SCOTUS has never really ruled on the near ban on automatic weapons.