Here's another source with individual mass shooting incidents and the type of weapon used. Count the number of incidents where an "assault rifle" was used.
I'll be generous and let you define all shotguns and all semi-automatic rifles as "assault rifles" even though nobody in their right mind would do so. I'll let you count every incident where a handgun was used with another weapon (a rifle, shotgun, knife, flamethrower, etc) as an "assault rifle" incident. Also, go ahead and count the incidents where the weapon was unknown as an "assault rifle" incident.
Now, count the number of incidents where only handguns were used. Add together the number of people who were killed in handgun only incidents and "assault rifle" incidents.
Now explain to me how it's obviously absurd to say that an "assault rifle" ban would not have helped all those people killed with handguns.