If someone is too dangerous to own a gun then they're too dangerous to be loose in society anyway. They can always find some other means to hurt people.
Don't know, I'm not an expert on that. But locking them in a dark room somewhere out of sight isn't the answer. The whole "until they receive sufficient treatment" is the missing piece in the puzzle, because that doesn't happen. That's the part I'd like to see get fixed instead of spending money on torturing them.