The boy is recovering at Children's National Hospital.
Police say he was playing with his sister when they found the gun in the parking lot off of West Franklin Street, which is also behind John Wesley United Methodist Church. Police say the children were playing with the gun when it fired. The boy's sister immediately ran to her home and told her parents what happened.
The boy was rushed to Meritus Medical Center around 11:30 a.m.
The gun was found near the lot. Police found the gun unloaded.
"There possibly was a gun laying here that the juveniles found," says Jim Hurd of the Hagerstown Police Department. "They were playing outside. A 3-year-old and a 7-year-old. They found the gun, manipulated it. The gun went off and struck one of the children in the chest."