What sort of test do you propose that those who steal guns or buy them on the black market to carry out their criminal acts take?
Without gun shows, the black market would be very scarce.
Increase the penalty on selling to a criminal, vigorously enforce the law by increasing the budge of the ATF.
Most NRA gun owners have the legal guns and then a few illegal guns off the books tucked away. Started raiding some homes, and if they find one illegal weapon, then the person looses the right to ever own a gun again.
We label sex criminals, why don't we label people who are not allowed to own guns?
The problem with you NRA nuts is you want to do nothing while kids are being murdered execution style.