Basically people of subnormal intelligence cannot purchase firearms, so your scenario of the "retard" should never happen because they are not allowed to purchase firearms.
Not all firearms are legally purchased and some times normal people flip out.
Used to be that if someone was crazy a friend, relative, neighbor could file a report, the cops
would come out and interview the subject. If the cops concluded the subject was crazy he/she
would be taken to a mental institution for evaluation. After x amount of time the subject would
appear before a judge and then anything could happen. See the movie Harvey for an example.
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