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Where did you get that absurd idea? Have you any idea how awkward it is to shoot yourself in the mouth with a rifle -- with your shoes on?
Besides. did you ignore the bloodstained revolver laying on the floor above the perp's head?
Some have said it looks like a Taurus "Judge", which has a 2.5" long cylinder that can hold .45 Colt rounds or 2.5" shotgun shells -- or a mix of the two.
My guess is that he shot himself in the mouth with a .45 Colt round. A .410 shotshell would have left relatively little intact brain tissue to autopsy...
The claim that there was an examinable brain left suggests that he directed the muzzle straight back -- which would have disconnected his Medulla oblongata, shutting his body down instantly. With an upward, (roof of mouth) shot, the brain would have been pretty badly disrupted -- even with the .45 Colt slug...
I thought that too from looking at the photo.
But one of the cops who rushed the room said on 60 minutes he looked at the body and saw no obvious head wound. He said he did see blood coming out of his mouth.