The statistics are cooked. Vox defined "mass shooting" as any case in which four or more victims were shot, and came up with 353, or almost one a day. USA Today counted only shootings in which four or more were killed, and came up with 29. Mother Jones counted only incidents like San Bernardino, Newtown, Ft. Hood, Columbine, and Virginia Tech, and came up with four a year.
The Vox statistics include gang turf wars, robberies, etc. So when T-Bone shoots rival gangsters Ice Dawg, JJ, and Ray Ray in a drive-by, and one of them shoots back and wounds him, Vox counted it as a mass shooting. Of course, the gang members are always convicted felons and/or juveniles, who are already legally barred from owning guns.
There have been a lot of attempted mass shootings where armed good guys stopped the killers before the death toll reached four, so they were not counted as mass shootings. (Pearl, MS; New Life Church; Garland, TX.)
In terms of real mass shootings like San Bernardino, Newtown, etc., every one of them took place in a designated gun-free zone.