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[A] public policy of simply discouraging people from owning or using firearms is not, in and of itself, a constitutionally permissible objective, any more than discouraging people from religious observance would be permissible to some oh-so-progressive government that considered religion as hopelessly declassé as progressives nowadays consider the right to keep and bear arms .... And any statute or regulation that burdens the right to keep and bear arms on the ground that guns are a public health hazard should enjoy the same frosty reception in court that would be given a statute or regulation that burdened the free exercise of religion as a mental hazard.