In the US, a person has to be adjudicated as mentally defective by a medical body to strip them of their guns. Under the current DSM, they don't count Transsexualism as a defect anymore. I don't agree but we probably won't be seeing that changed.
I wouldn't consider the WP leanings to be a defect. That is a slippery slope. Does one disallow Communists, Jews, Radicals, etc....?
Whether we in the US dislike it or not, the tranny was within his rights up until the shooting.
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What I do wonder about is if he lied on any of the forms, etc.....?