I'm going to say something that people aren't going to like but even in the mid to late 18th Century, the Thirteen Colonies (especially Massachusetts) had more firearms per capita than many European Countries. There has always been gun violence it isn't new what is new is people shooting up public places in such great numbers.
https://scholarship.law.wm.edu/cgi/v...9&context=wmlr
http://willamette.edu/law/resources/...son-6-2-08.pdf
https://ir.lawnet.fordham.edu/cgi/vi...21&context=flr
* Note the above links are PDF formate academic journal articles.