Did not semi-automatic weapons exist before 1970? 
Did not high capacity magazines exist before 1970?
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So what happened....sometime before or after 1970.....that caused the current "uptick" in "mass shootings" where 24 of the Top 25 Mass Shootings in the US did not occur in the Wild West of the !800s where a lot more people carried more guns; nor in the Gangster Era of the 1920s and 1930s; nor at ballparks, concerts, movie theaters, and the like in the 1950s and 1960s where metal detectors did not exist and no one knew - or gave much thought to - the idea that perhaps a couple hundred people at a ballpark might be carrying a gun......and no "mass shooting" took place that caused the hysteria it causes today? 
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Something happened. Something changed. Something happened around 1970 that caused a "shift" in the fundamental thinking of our society that led to this uptick in the "sudden surge" of mass shootings in "this generation" and not before "this generation."