"Police did not know what kind of gun was used."?
First of all.. I would suspect it was a pistol, based on my suspicion that it would be not only more difficult to shoot yourself in the head with a rifle, but that one of the hospital security guards might notice a person strolling in with a rifle.
Secondly, shouldn't someone trained to use a gun have a clue as to the caliber pried from a dead man's hand might be, based on both the bore of the barrel and the size of the hole in the person?