In most cases, just engaging a mass shooter will stop the killing because most of them are not mentally prepared for a gunfight. That said, carrying a gun for self defense is very different than being a LEO. The average armed citizen is under no obligation to confront a mass shooter and most probably wouldn't unless there is no other choice. You would want to have good handgun skills to go up against a rifle. Ultimately, it depends on the individual and the exact situation.