It's well worth noting that there is a solid history of claims of “mental illness” being used by tyrannical governments to deprive dissidents of basic rights, over political disagreements. The Союз Советских Социалистических Республик was particularly notorious, having a number of психушка, or “mental hospitals” that were really prisons for political dissidents.
Think it can't happen here? Have a look at
this thread. The OP of
that thread may be a bit of an extremist, but the idea is already gaining traction that those who hold to certain essential values of morality, and refuse to go along with those who demand that society embrace certain forms of madness and immorality, are to be marginalized as “bigots”, and possibly even as “mentally ill”.
Is it really so difficult to believe that prohibitions against “mentally ill” people possessing arms wouldn't be abused by including as “mentally ill” those who hold entirely-valid opinions that are deemed politically-incorrect?