Father, cleaning gun, killed 12 year old son.

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I just read that a guy in Michegan was cleaning a gun he had taken from storage, he forgot there was a round in it, and it went off and killed his 12 year old son, who had stayed home from school sick. I lived in an apartment once, the guy below me fired a shot up through the floor, in my kid's bedroom (luckily he wasn't in there). Is cleaning a gun so difficult that the gun had to be fired while doing it? I have never owned a gun, so I don't know. It just seems like such a tragic thing to have happen for that family, and such a horrible thing the father must be going through.