I was engaged in firearms enforcement for more than 20 years, and in my experience, most guns for crime are obtained through secondary transactions. It is rare for a person to purchase a firearm from a licensed dealer and shortly thereafter, use it to commit a crime.
Either Kahn’s understanding of history is flawed, or you threw out a red herring. The three-fifths clause was not a sociological or anthropological statement (i.e., slaves were only three-fifths of a human), but rather a political compromise struck between the northern and southern states for control of House seats and the levying of taxes based upon census. Without this clause, the Constitution would not have been ratified by the original states. It is far too simple to judge our founding fathers from the benefit of over 200 years of hindsight.
There are measures which can be implemented to prevent the sale of firearms to legally prohibited persons and to remove firearms, temporarily or permanently, from persons who pose a public safety risk. These measures require legislative action; however, no one should assume that legislative action alone will eliminate gun violence. Criminals, by nature, do not obey laws and do not voluntarily surrender the tools of their trade. There will always be a deranged person ready to strike at a soft target.
We harden facilities which house valuable assets, such as banks, data centers, and warehouses. Why have we not hardened facilities which house our most valuable assets: our children?