All of the weapons used in mass shootings were legally obtained by the shooter or someone close to the shooter. An ATF gun license system such as a drivers or pilot license needs to be enacted.
A free basic gun license for hunting rifles, shot guns and revolvers with a basic level of training.
A Semi-automatic license. This would require more training, annual medical, psychological certification and a licensing fee of $20,000 per year. Annual registration and inspection for weapons would be required.
Current gun owners would have to abide by the new licensing requirement. If they choose not to obtain such a license, they may opt to a government buyback or have the weapon be made permanently inoperable.
Violation would result in a felony and substantial fines.