The pistol he had was a revolver without a safety or hammer. It is assumed, that he had the revolver in case someone came to the door while he was moving the rifles. The theory is, he was holding the revolver in his hand then moving 4 of the rifles to a new position. One of the rifles slipped and his automatic response was to squeeze his arms, which also made him squeeze the trigger of of the pistol. The shot that killed him was into his mouth and he died by choking on his blood. If he really want to kill himself and had planned this all out, he probably would have aimed a little better with his last shot.