Even if you limit "mass shootings" to incidents with at least ten fatalities, you would have to eliminate at least twelve incidents from 1949 to 1999. It's hard for me to see any plausible single POD for doing this. (No, "outlaw all guns"--even assuming it would have prevented all these incidents--is not plausible.)
Camden shootings
1949 13 3 Semi-automatic pistol
[61][62]
University of Texas tower shooting
1966 18 (inc. the perp.)
[fn 1] 31 Rifles, revolver, pistols, and shotgun
[53]
Easter Sunday massacre 1975 11 0 Semi-automatic pistols and revolver
[73]
Wilkes-Barre shootings 1982 13 1 Semi-automatic rifle
[63][64][65]
Wah Mee massacre 1983 13 1 Semi-automatic pistol(s) and/or revolver(s)
[fn 2] [66]
San Ysidro McDonald's massacre
1984 22 (inc. the perp.) 19 Semi-automatic rifle, pistol, and shotgun
[53]
Palm Sunday massacre 1984 10 0 Semi-automatic pistols
[77]
Edmond post office shooting 1986 15 (inc. the perp.) 6 Semi-automatic pistols
[53]
GMAC shootings 1990 10 (inc. the perp.) 6 Semi-automatic rifle
[53]
Luby's shooting
1991 24 (inc. the perp.) 27 Semi-automatic pistols
[53]
Columbine High School massacre 1999 15 (inc. both perps.) 24 Semi-automatic rifle, semi-automatic pistol, shotguns
[57]
Atlanta shootings 1999 10 (inc. the perp.) 13 Semi-automatic pistols and revolver
https://en.wikipedia.org/wiki/Mass_shootings_in_the_United_States