Mental illness is a tricky one. HIPPA will prevent disclosure to most others and most agencies. Are spouses, parents, or adult children even allowed to know that someone you know is diagnosed ?
When the state starts to dictate who should not own a firearm (based on mental illness) … a new slippery slope is uncovered…
Legally purchasing and possession of firearms vs. simply possessing firearms (illegally) is another situation that presents some rather high hurdles to effectively enforce. Most criminals simply possess firearms via illegal purchase … no background check needed.
Simple random stops while extremely unconstitutional did likely help reduce the amount of illegal firearms out there … whole other discussion though, right ??