Circumstances change, that's the main fundamental issue. At the same time, we don't even check for problems during a lot of gun sales. I would say domestic abuse charges, history of suicidal behavior or terroristic threats, etc. There are at least lines we can draw without it being a free for all or a massive lockdown. Admittedly, it's mostly so we can feel we're at least trying something in the face of our children being killed that may actually make an impact in some of the examples.