An eight-year-old boy found a double-barrelled 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 said: "To find a gun in a place where children play, in a residential area, is totally unacceptable."
"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."
Police said the boy found the gun in a plastic bag on some grassland. He then took it home to his parents who called the police.
Ch Insp Jerry Harris told the BBC: "I understand that it's a 12-bore, double-barrelled shotgun."
Forensics examinations are now being carried out on the weapon to see if it can be linked to other firearms incidents.
Ch Insp Harris added gun crime was a "serious worry" at the moment.
"The implications are very serious... people who are carrying out their nefarious activities, armed with firearms, are completely and utterly disrupting the fabric of society, of the good communities where they live," he said.