Someone should also point out that high capacity magazines for semi auto handguns have been available since WW1, the Luger Snail Drum is a case in point, and as a standard feature for a 9mm handgun since 1935, the Browning HP.
Semi Auto carbines with 30 round magazines have been available to the public since the 1950's. Yet school shootings were practically unheard of before the late 90's.
Availability of the rifles is not the deciding factor, its the evil intent of a very few homocidal pyschopaths.
Mass murders have been performed using gasoline and improvised explosives, with no firearms of any sort involved.
Personally I don't have any pressing need or desire for an autoloading rifle of any sort, but theres a lot of things I don't need that others should be able to buy should they feel the need.
Anyone who has been stuck in a large city during a major riot could easily see the need for a high capacity autoloader to defend the lives of yourself and family and to protect your property.