As we get to about 2 feet away, the wound begins to look like a large central hole with a few little holes around the edge. Beyond 4 or 5 feet the shot disperse more and more and is more likely to make lots of little wounds, not likely to be fatal. Thus a wound that is five inches across was done at four yards. Surprisingly few gunshot fatalities are accidental. People who own guns are usually very careful with them.
Most accidents happen if someone else gets hold of the gun.
First, check to see if it is physically possible that the wound could be self-inflicted. If the gun was fired more than 3 feet away, then it can't be. A wound in the back of the head is more likely to be homicide than suicide. Some of these indicators are obvious at the scene. Only about a quarter of suicides leave notes.