United States police say a 4-year-old boy has accidentally shot and killed his father at a residence in the southwestern US state of Arizona.The Prescott Valley Police Department said the shooting happened just after Friday noon.The 35-year-old father, identified as Justin Stanfield Thomas, and his young son were staying at a friend's house, when the child got hold of a gun in the home's living room and accidentally discharged it.A bullet hit his father, an Iraq War veteran with the Army Special Forces, in the chest.Thomas was transported with life-threatening injuries to a nearby medical center, where he was pronounced dead upon arrival.The boy was taken to a police substation to undergo interviews. He is currently at home with his mother in Phoenix."He doesn't know that his father is gone," the boy's grandmother, He's still waiting to play with him some more."Marilyn Andreatta, said.Every year, more than 30,000 people are shot and killed in the United States.The US averages 87 deaths each day due to gun violence, with an average of 183 injured, according to the University of Chicago Crime Lab and the Centers for Disease Control.Nevertheless, the year 2012 was a record-setting year for gun sales in the US.About 4.5 million firearms are sold annually in the United States at a cost of 2 to 3 billion dollars.On December 14, 2012, twenty children and six adult victims were fatally shot by a gunman - who later killed himself - at the Sandy Hook Elementary School in Newtown in the US state of Connecticut. Earlier in the day, the assailant killed his mother in another location.