On the surface, that makes perfect sense.  I would want a strong provision in there somewhere that would specify the mental illness criteria so as not to exclude unnecessarily.  The vast majority of those seeking help from a mental health professional are not dangerous at all.  Some states require adjudication the same way you would need adjudication to hold someone involuntarily in a mental hospital.  There should be some due process before a right is stripped.  But let's be real, even then it may not have much of an effect as we have numerous examples of killers going around the background check process.  The San Bernanrdino shooters bought their rifles from a friend.  We can prosecute that friend now but it's little consolation.  The two Columbine teen shooters got around the law by having a friend buy them the weapons at a gun show (and that friend passed a background check proving that there really is no gun show loophole).  We should just have realistic expectations.  Cheers!