Compiling copious amounts of data, the non-profit Crime Prevention Research Center (CPRC) concluded that more than nine in ten mass shootings occurred in gun-free zones from 1950 through May 2018.
Hudson, Jerome. 50 Things They Don’t Want You to Know (p. 40). Broadside e-books. Kindle Edition.
Thank you for the reference!
Here’s link to the research & data: