Most of the mass shootings have came since we started closing mental institutions and stopped committing people that were insane.
The committment proceedings are medical, overseen by judicial decree to ensure the committed are being fairly evaluated. This worked for decades until the psychologists and social workers thought violently insane people could be treated.
The last five big shootings, Ft. Hood, Columbine, Aurora, Arizona and Newtown were by people that had recently received psychiatric treatment, psychotropic drugs or Muslim fanatics.
How is decreasing magazine size (can be changed in a second) going to address that problem?
Put the insane away before they kill more. They love "gun free zones" as target rich areas.