mentally defective can't possess firearms.
Because all the other laws restricting it aren't working.
Maybe we should just make it harder for law-abiding citizens who are responsible gun owners to obtain firearms. That will certainly make it harder for criminals to get guns.
http://www.stltoday.com/news/local/crime-and-courts/laclede-gas-shooting-victim-was-outdoorsman-killer-had-criminal-past/article_b56eac4d-cbc6-5a7f-9214-e63d5ab6c337.html