That depends on how you define "mentally challenged." If someone is elderly and merely needs help managing their financial affairs, then no, I don't think that person poses any danger and should not be stripped of their 2nd Amendment rights. That was what was at issue when Congress recently overturned the Obama administration's expansion of prohibited purchasers under the Social Security rules. Most of what you suggest as "rational reform" is flatly unconstitutional. Semi-automatic rifles and handguns (military style or otherwise) have been around for more than a century and are in use by tens of millions of Americans. Therefore they are considered arms "in common use" and are protected. So-called universal background checks are not really universal because there's no way to enforce them without a massive federal registry, which is illegal. In addition, gun control advocates cannot point to any high profile shootings that would have been stopped by background checks on private sales.