Wow ... This is such a blatant LIE.
BATFE Form 5300.9 MUST be completed to legally buy a firearm.
It specifically asks if the applicant has been adjudicated mentally defective, or been committed to a mental hospital.
If so - the applicant is denied the ability to legally purchase a firearm.
Furthermore, most states require a background check in order to purchase a handgun. Some to purchase a rifle.
One of the things the states check, is if the person has a criminal record - or been committed to a mental hospital or is "mentally defective".