Where'd you get that?  The Homicide rate (murder and non-negligent manslaugher) in 1950 was 4.5 per 100,000.  It peaked in 1980 at 10.2, and has been in a steady decline since, and gun ownership grew.  In 2014, it was back to 4.5.

Up to 80% of gun homicides in this country are gang related, either black on black (55%), or Hispanic on Hispanic, and occur in the inner cities.  The majority of the victims have a criminal history themselves.  It doesn't pay to not pay the man when the drug money is due. The late payment penalty is pretty serious.  Gotta send a message to the others, ya know.

https://www.infoplease.com/us/crime/homicide-rate-1950-2014

Deaths from mass shootings are such a small percentage of the total, they're almost irrelevant.