A gun and ammunition were found by two children playing, police have said.
The firearm, thought to be a self loading pistol, was discovered on an embankment in Bury.
Greater Manchester Police said the two 13-year-old boys had been playing in the area when they found the items in a bag on Bentley Lane, near the M62 motorway.
The gun had six bullets in the magazine, and officers recovered a further 26 rounds of ammunition.
A spokesman said the children alerted police and officers from GMP's Xcalibre Organised Crime Unit attended and recovered the firearm.
Inquiries are continuing to establish who hid the gun.
Detective Sergeant Dave Caulfield, from GMP's Xcalibre Organised Crime Unit, said: "Finding a firearm is always a great result but the circumstances surrounding this recovery are extremely concerning.
"Whoever has dumped this clearly has no regard for human life as this weapon, in the wrong hands, could have killed someone.
"Thankfully the children contacted us and we are now working to establish who left the gun there so if anyone has any information that can help I would urge them to get in touch."
Anyone with information is asked to call the Xcalibre Organised Crime Unit on 0161 856 6436, or the independent charity Crimestoppers, anonymously, on 0800 555 111.