Why was the first recorded mass shooting in The USA the same year as the Thompson Submachine Gun becoming retail available?

'Firearms aren't the problem and firearm related deaths have actually lowered'

Even though statistics show otherwise
Update: https://en.wikipedia.org/wiki/Mass_shootings_in_the_United_States#Deadliest_mass_shootings_since_1949

Take a drink every time Semi-automatic comes up.
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