there doesn't seem to be much evidence that gun control affects the ability of criminals to get guns.

Mass murderers such as school shooters aren't really typical criminals, though. They're very unusual criminals. Do they have access to black-market guns the way that career criminals might?

Some school shooters (e.g. Wayne Lo, Seung-Hui Cho) bought their guns legally. Others (e.g. Adam Lanza) used guns belonging to family members. Harris and Klebold had another person purchase guns legally for them, but also purchased one gun illegally. Kip Kinkel was given guns by his parents, but also bought a gun that another student had stolen from a friend's father.