The nine-month-old infant was killed in his playpen when his five-year-old brother found a loaded Magnum gun in the home and accidentally fired it.
The five-year-old discovered his grandfather's .22-calibre magnum that was kept on a shelf built into the headboard in the master bedroom, Nodaway County Sheriff Darren White told The Washington Post.
The baby was lying in a crib in the same room when the gun went off, and a bullet struck him in the head.
"It was just lying out," White said of the gun. "Little kids are curious. It was in the headboard of the bed, and he found it."
The children, their mother and two other siblings were reportedly staying in their grandfather's home in Elmo, Missouri, according to a neighbor.
The baby boy was rushed by air ambulance to a hospital in Kansas City, where he was pronounced dead.
The mother, Alexis Widerholt, was in the kitchen when she heard a loud noise that sounded like a gunshot, according to White. She made a frantic call to emergency services who came to the house in Elmo, in northwestern Missouri.
Initial reports stated that the woman told 911 that her son had shot his younger brother in the head with a paintball, said Sheriff Darren White.
However, when medical crews turned up at the tragic scene they found out that the baby had been shot in the head with a .22 calibre Magnum revolver.
"You know, just some gun safety could have come into play, and would have maybe, you know prevented the situation, but it's just really sad, altogether," neighbour Jessica Hutchison told KCTV.
Gun control advocates estimate that nine children are unintentionally shot every day in the United States.
Police believe the shooting was accidental, although the incident is still under investigation.