The shooting occurred Thursday night in the Phoenix suburb of Peoria while the boy and a 7-year-old brother were being watched by a grandmother and aunt. The aunt was washing dishes in the kitchen when the gun went off, according to an audio recording from a 911 call released by police.
Police say the boy was playing on the bed in the master bedroom when he found the gun, which belonged to his father, who was not at home at the time.
The bullet struck the boy's cheek and exited his head, authorities said. A portion of his skull was removed to alleviate swelling.
Peoria police spokeswoman Shari Howard identified the father as Manuel Zeschitz-Garrison. He has been cooperating with the investigation, which is still ongoing. No criminal charges are pending at this time. The boy is not being identified because he is a juvenile.
Under Arizona state law, incidents where a child is put in a situation that could cause injury or death can result in child abuse charges.