An eight-year-old boy found a shotgun on waste ground "where children play" in Liverpool.
Police were called at about 18:00 BST on Sunday to reports a child had discovered a gun near Church View and Chapel Terrace, in Bootle.
Merseyside Police is carrying out house-to-house inquiries near the scene and studying CCTV.
Ch Insp Steve Rice said there could have been "an absolute tragedy" and described it as "truly shocking".
He added: "To find a gun in a place where children play, in a residential area, is totally unacceptable."
Ch Insp Rice said the boy went to find an adult who then alerted police.
"Luckily this gun was not loaded - I dread to think what could have happened to the innocent young boy who found it," he said.
"We could have been facing an absolute tragedy today following the reckless actions of the thoughtless individual who left the gun in a place where children play."